/ INTRODUCTION TO THESTUDYOF HEREDITY f * Y^ T~\ i^t i""* T\ nr^ r^ ^ " F r r i 1 ' i~^ i~\ i: :RB ERIE. WALTER Marine Biological Laboratory Library Woods Hole, Massachusetts Gift of F. R. Lillie estate - 1977 r-=1 D D a m a GENETICS THE MACMILLAN COMPANY NEW YORK BOSTON CHICAGO DALLAS SAN FRANCISCO MACMILLAN & CO., LIMITED LONDON BOMBAY CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, LTD. TORONTO GENETICS AN INTRODUCTION TO THE STUDY OF HEREDITY BY RBERT EUGENE WALTER ASSOCIATE PROFESSOR OF BIOLOGY BROWN UNIVERSITY Nein THE MACMILLAN COMPANY 1917 All rights reserved COPYBIOHT, 1918, BY THE MACMILLAN COMPANY. Set up and electrotyped. Published February, 1913. Reprinted June, October, 1913; February, July, 1914; July, December, 1915; July, 1916; March, 1917. ? 'c? Nortooob J. 8. dishing Co. Berwick 1 .72, FORMULAE: A.D. = - C.V= ATFr Ssum x- deviation of the class from A.M. f - number in the class n- total number Number of Rays 3 4 5" 6 7 FIG. 25. The fluctuating variability of starfish rays. From data by Goldschmidt. eighteen as the commonest kind ; in the second case, around fifteen. Such a difference could not easily VARIATION 45 be detected or expressed by any other method than the statistical one. Again, in the case of forty-seven starfishes all of which were collected from one locality the variation in the number of rays proved to be, according to Goldschmidt, an amount indicated graphically in Figure 25, where the data are arranged in the form of a so-called frequency polygon or curve. From such a polygon certain constants may be computed which conveniently express in a single number, for purposes of abstract comparison, dis- tinctions that otherwise could be handled only in the most indefinite way. Thus in this instance the arithmetical mean, ex- pressed by the hypothetical number 4.915, a number which of course does not actually occur in nature, is simply the average number of rays in forty-seven starfishes selected at random. The mode which represents the group containing the largest number of individuals of a kind, namely, thirty out of forty-seven, is five in this particular polygon. The average deviation, which is an index of the amount of variation going on among the starfishes in question, is .52. In other words, .52 is the average amount that each individual starfish deviates from the arithmetical mean of 4.915. Although the one seven-rayed starfish which happens to be in the lot varies from the standard of 4.915 to the extent of 2.085 (7 4.915) rays, there are thirty five-rayed starfishes which vary only .085 (5 4.915) of a ray, 46 GENETICS and consequently the average of the entire forty- seven amounts to .52 of a ray. In another collec- tion of starfishes where either more seven-rayed or two-rayed specimens might be present, the average deviation would probably be greater. By computing the average deviation, therefore, and using it as the criterion of variation, a compar- ison of the variability of organisms that have been taken from different localities or subjected to differ- ent conditions can be definitely expressed. A measure of variability more commonly in use by biometricians, since for mathematical reasons it is more accurate, is the standard deviation. This is the square root of the sum of all the deviations squared and their frequencies divided by n, accord- ing to the formula i v / 2 n /-*\? 'J) n in which x represents the deviation of each class from the arithmetical mean ; /, the number of individuals in each separate class ; 2, the sum of the classes ; and n, the total number of individuals. 1 In the present instance the standard deviation is .846, an arbitrary number that has valuable sig- nificance only when brought into comparison with standard deviations similarly derived from other groups of starfishes. Such a variation polygon as the above expresses the law that the farther any single group is from the 1 For directions explaining the use of such formulas see Davenport's " Statistical Methods." VARIATION 47 mean of all the groups making up the polygon, the fewer will be the individuals that represent it. 7. THE INTERPRETATION OF VARIATION POLYGONS a. Relative Variability The statistical determination of the relative vari- ability of two lots of organisms with respect to a certain character may be illustrated by the case of the oyster-borer snail, Urosalpinx cinereus, as seen in the accompanying table. ATLANTIC AND PACIFIC SHELLS COMPARED LOCALITY NUMBER OP SHELLS A.M. O 65 70 75 80 85 9O 95 < Ratio of height of head to length of shell FIG. 29. Schematic curve of the head height of Hyalodaphnia under various conditions of nourishment. Adapted from Woltereck. cording to the Lamarckian school, in the "environ- ment" and "training" sides of the triangle of life rather than in the "heritage" side (Fig. 1). For example, Woltereck, by controlling the single 54 GENETICS extrinsic factor of food supply, was able to modify the height of the "head" of the microscopic fresh- water crustacean, Hyalodaphnia, in the remarkable manner indicated in Figure 29. When poor food FIG. 30. Variations in the number of stamens in the flowers of the " live- for-ever" (Sedum spectdbile) under various controlled conditions. For detailed description, see text. After Klebs. was supplied, the percentage of the head height to that of the body averaged hardly forty, while with rich food it was increased to over ninety. Similarly Klebs succeeded in changing at will the number of stamens in the common " live-for-ever," Sedum spectabile, by manipulating the environment in which the plants were kept. Some of his results are shown in Figure 30. Polygon A combines the data for 4260 flowers which were raised in well-fer- tilized dry soil under bright light ; polygon B repre- sents 4000 flowers grown in a moist greenhouse under red light ; and polygon C includes 4390 flowers VARIATION 55 from well-fertilized soil in moist hotbed conditions under a weak light. c. Weismann, on the contrary, believes that the causes of variation, at least of heritable variations, are intrinsic or inborn in the germplasm. His con- ception of sexual reproduction is that it is a device for doubling the possible variations in the offspring by the mingling of two strains of germplasm (am- phimixis). By far the greater number of observa- tions recorded go to substantiate this theory. Tower found among his potato-beetles, for exam- ple, that two strains reared in the same environment showed striking differences in variation, a fact necessarily due to intrinsic rather than to extrinsic factors. Similar cases may be recalled by any one. d. Lastly, Bateson, whose work "On Materials for the Study of Variation" already cited is a classic, takes the agnostic attitude that it is rather futile to guess at the causes of variation before the facts are well in hand. He consequently discourages such attempts by saying : "Inquiry into the causes of variation is, in my judgment, premature." In conclusion, the words of Darwin written half a century ago "Our ignorance of the laws of variation is profound" may still be appropriately quoted, notwithstanding the fact that in biometry we have at least an excellent analytical method by means of which considerable insight into variation is being gained. CHAPTER IV 1. THE MUTATION THEORY AMONG the possible kinds of variation already hinted at are so-called mutations which are clearly defined from the fluctuating variations to which reference has just been made. Darwin was fully aware of the existence of muta- tions or "sports" and incidentally gave time to their consideration, but the great task which he accomplished in such a masterly manner was to overthrow the widespread and deep-seated belief of his day in a sudden special creation of distinct species. To this end he marshaled evidence in support of the gradual transition of one species into another, emphasizing fluctuations rather than muta- tions which seemed to him to play a minor r61e in the origin of species. It remained for the Dutch botanist Hugo de Vries to analyze the character of mutations. There is something distinctly suggestive of Darwin's method in the fact that de Vries worked in silence for twenty years before he gave to the world the "Mutations- theorie" with which his name will forever be con- nected. 56 MUTATION 57 2. MUTATION AND FLUCTUATION A mutation is something qualitatively new that appears abruptly without transitions and which breeds true from the very first. To use the musician's phraseology, it is not a variation elaborated upon an old theme, which would correspond to a fluctuating variation, but it is an entirely new theme. The difference between mutations and fluctuating varia- tions is generally not one of quantity or magnitude, although it sometimes may be so, since muta- tions are often much smaller than fluctuations. Mutations are discontinuous in the same sense that chemical combinations, such as carbon monoxide (CO) and carbon dioxide (CC^), are discontinuous, but the leap from one to the other may be so small that frequently it is difficult to ascertain by inspec- tion alone whether the difference is due to a mutation or a fluctuation. The test comes in breeding,. for the progeny of a fluctuation will vary around the old average of the parental generation, while the progeny of a mutation will vary around a new average, set by the mutation itself. When a series of mutations is treated statistically, it does not arrange in frequency polygons as readily as a series of fluctuations do. The latter mass around the average standard according to the laws of chance much in the same way that a hundred shots by a good marksman may center around a bull's- eye. Mutations never act in this way. They find no correspondence even with wild shots at the bull's- 58 GENETICS eye. They are shots directed at a different target altogether. To the student of heredity there are two distinc- tions of prime importance with respect to mutations. First, that they usually appear full-fledged without preparatory stages, and second, that they breed true from the start. Fluctuations, on the contrary, ordin- arily "revert" to the parental type in subsequent gen- erations. The great practical importance to the breeder of a knowledge of these heritable mutations is at once apparent. 3. FREAKS /A further distinction should be made between mutations and so-called freaks or monstrosities, namely, that the former breed true, while the latter do not. A human physical deformity, such as a club-foot, for example, or a humped back, is not a mutation, because it does not reappear as a heritable character. Variations of this kind are not predeter- mined in the germplasm, but are usually instances of something that went wrong during the development of the individual somatoplasm. Thus, among normally "right-handed" snails "left- handed" individuals have occasionally been dis- covered which, when bred, were found to produce all normal "right-handed" progeny. They are therefore not mutations at all, but freaks or mon- strosities due probably to some unusual occurrence early in the cleavage stages of the embryo. MUTATION 59 4. KINDS OF MUTATION De Vries has classified mutations according to their component units into three categories: pro- gressive, regressive, and degressive. Progressive mutations are signalized by the addi- tion of a new character to the sum of complex char- acters making up the individual. If rumor may be believed, Anne Boleyn, the second in the interesting series of wives of Henry VIII, was a progressive mu- tant with respect to at least one character, for she is said to have possessed an extra finger on each hand, as well as the abnormalities of supernumerary mammae and extra teeth. Evidences that each of these three characters occur as heritable mutations is presented in Davenport's " Heredity in Relation to Eugenics." Regressive mutations are characterized by the dropping out of something. Thus albinism is caused by the absence of pigment or color. Albinic mutants which breed true are well known, particularly among mammals, such as rats, mice, rabbits, cats, guinea- pigs, and even man himself. Degressive mutations include cases of the return of a character which was formerly present in the past history of the race, but which has for generations been absent or latent. Castle's four-toed race of guinea-pigs furnishes an example of this class of mutations. In 1906 Professor Castle discovered a newly born guinea-pig in one of his pens with four toes on each hind foot, from which he has successfully established a four-toed race. The hypothetical an- 60 GENETICS cestor of the rodents probably had five toes on each foot, but the normal number in modern guinea-pigs is four on each of the front feet and three on the hind feet. The individual from which Castle has bred a four-toed race exhibited a degressive muta- tion, tending toward the ancestral type. 5. SPECIES AND VARIETIES The doctors have always disagreed regarding a definition of species. What determines the exclusive boundaries that shall isolate from their fellows any particular group of animals or plants has long been a mooted question, and still remains so. The Linnsean concept of a species was that of an exclusive caste of individuals, inflexibly demarked, over whose high barriers no nondescript tramps would dare attempt to climb. When an entomolo- gist of the old Linnaean school encountered an insect which did not conform to the morphological tradi- tions of its fellows, the frequent fate of such a non- conformist was to perish under the boot-heel rather than to find sanctuary in the cabinet of the pre- served. Since it was an exception, and a violator of the divine law of the fixity of species, it deserved to be annihilated! Those were hard days both for heretics and for mutations. The method of the older school of systematists may be described as one which emphasized differences and put up barriers that should keep the unlike apart, at the same time allowing only "birds of a feather to flock together." It was a brave and sue- MUTATION 61 cessful attempt to bring order out of chaos by classi- fying the living world, and it served its purpose well until Darwin's idea of half a century ago, that the origin of all species is from preceding species, put an entirely new face upon the whole matter. Organ- isms of different species were found to be related to one another, and even man could no longer escape acknowledging his poor animal relations. As a consequence, likenesses rather than differences there- after claimed the most attention. During the reconstruction of phylogenetic trees, which seized the imagination and became the prin- cipal business of biologists as soon as the ** Origin of Species " was made common property, the crotched sticks in the woodpile of organisms, that had hitherto been largely discarded, were most eagerly sought after. It was just these scraggly sticks, that were neither trunk nor limb-wood but combinations of both, which told the story of continuity and were indis- pensable in building up a reunited whole. As the analysis of the living world gradually came to shift from species to individuals, it was shown that individuals may be regarded simply as ag- gregates of unit characters which may combine so variously that it becomes more and more diffi- cult to maintain constant barriers of any kind be- tween the groups of individuals arbitrarily called "species." The old species of the systematist, upon analysis into their respective unit characters, dissolve into numerous "elementary species" and "varieties" dif- 62 GENETICS fering from species perhaps only by the addition or subtraction of a single character, and thus the FIG. 31. Diagram to illustrate various ideas about "species." Under Species A are represented two groups of individuals which are near enough alike to be placed within a single species, but which are suffi- ciently unlike each other to constitute the " sub-species " or "varie- ties" of Darwin. Under Species B are various groups of individuals distinguished from each other by the addition or loss of one or more characters. These groups represent the "elementary species" and "varieties" of de Vries. "The "barrier of Linnaeus" attempted to separate species absolutely from each other. Darwin sought to find loopholes in this barrier. To-day attention is directed rather to the relation between individuals than to the boundaries between species. possibilities of analytical classification have become almost limitless. An elementary species, according to de Vries, is a progressive mutation differing from the type species MUTATION 63 by the addition of at least a single character, while varieties are regressive mutations distinguished from the parent type by the loss of at least one character. Both breed true to their respective modifications. These different concepts of what constitutes a species, illustrated diagrammatically in Figure 31, pave the way for a better understanding of muta- tions in connection with heredity. 6. PLANT MUTATIONS FOUND IN NATURE The oldest known authenticated case of a plant mutation is the often cited instance of the "fringed celandine," Chelidonium laciniatum, which made its appearance in the garden of the Heidelberg apothe- cary Sprengel in 1590 among plants of the "greater celandine," Chelidonium majus. The fringed cel- andine bred true at once and is now a widespread and well-known species. The purple beech has appeared historically as a mutant among ordinary beeches upon at least three occasions in widely separated localities, and it has always given rise to a constant progeny. The "Shirley poppy," notable for its remarkable range of color, originated from a single plant of the small red poppy, Papaver rhceas, which is commonly found in English cornfields. Instances are known of double flowers among roses, azaleas, stocks, carnations, primroses, petunias, etc., arising from single flowering plants, the seeds of which in turn produce double flowers. 64 GENETICS 7. LAMARCK'S EVENING PRIMROSE The most widely known plant mutations are the progeny of Lamarck's evening primrose, (Enothera lamarckiana, because it was these plants that led de Vries to formulate his mutation theory. It is believed by botanists in general that this plant is a native of the southern United States, al- though it is now, so far as is known, extinct as a wild species in America, and native specimens are included in but few American herbaria. It was exported to London as a garden plant about 1860, and from thence it spread to the continent, where, escaping from gardens, it became wild in at least one locality near Hilversum, a few miles from Amsterdam. Here, in an abandoned potato field, it fell under the seeing eye of Hugo de Vries in 1885, and now both botanist and primrose are famous. De Vries found among these escaped plants not only 0. lamarckiana, but also two other kinds or mutants, 0. brevistylis, characterized by short- styled flowers, and 0. lavifolia, which has smooth leaves. These two were entirely new species hitherto unknown at the great botanical clearing-houses of Paris, Leyden, and the Kew Gardens. Since the seeds of the (Enothera are produced by self-fertilized flowers, de Vries felt safe in regard- ing these plants as mutants rather than hybrids, and he continued to study them with especial care. Transplanting the mutants along with representa- tives of 0. lamarckiana to his private gardens in MUTATION 65 Amsterdam, where it was possible to maintain them in normal healthy condition, de Vries was able to follow their individual histories with certainty. He found that, out of 54,343 plants of the species 0. lamarckiana grown during eight years, there ap- peared 837 mutants comprising seven different ele- mentary species, all of which, with the exception of 0. scintillans, bred true. See table. MUTANTS OF (ENOTHERA LAMARCKIANA 2 < a i GENERATION 0) Q o z o I E S I | | fc TOTAL o 5 J ffl s 25 4 3 i 3 O o o ft fc 1-1 GQ I 1886-7 9 II 1888-9 15,000 5 5 III 1890-1 1 10,000 3 3 IV 1895 1 15 176 8 14,000 60 73 1 V 1896 25 135 20 8,000 49 142 6 VI 1897 11 29 3 1,800 9 5 1 VII 1898 9 3,000 11 VIII 1899 5 1 1,700 21 1 1 56 350 32 53,509 158 229 8 54,343 Some explanatory comment on this table may be of value. The seeds in each generation were self-fertilized lamarckiana. The mutant gigas occurred once, in 1895. From the seeds of this one plant were produced 450 true gigas offspring in the first year, and the strain con- tinues to breed true. Albida was first noted in 1895, but de Vries remem- p 66 GENETICS bered having seen it before and dismissing it as pathological. Because of its poverty in chlorophyll it is a mutant which probably would not maintain itself successfully in nature, but it breeds constant under cultivation. Oblonga always bred true with the exception of throwing an albida in 1895 and a rubrinerms in 1899. Of rubrinervis over 2000 invariably bred true, while nanella bred true in over 20,000 offspring, with but three exceptions when oblonga characters appeared. Lota, since it produces only female flowers and so cannot be self-fertilized, had constantly to be crossed back with the parent lamarckiana, when it produced from 15 to 20 per cent lata and 80 to 85 per cent lamarckiana. Finally, scintillans which appeared at three separate times proved constant only in its inconstancy be- cause it invariably produces a heterogeneous progeny. The 1895 plant gave 53 per cent lamarckiana, 35 per cent scintillans, 10 per cent oblonga, and 1 per cent lata. One of the 1896 plants gave 51 per cent lamarckiana, 39 per cent scintillans, 8 per cent oblonga, 1 per cent lata, and 1 per cent nanella, while another 1896 plant gave only 8 per cent lamarckiana, but 69 per cent scintillans, 21 per cent oblonga, and 2 per cent of nanella and lata together. These seven elementary species are distinguished from each other by features which are unmistakable even to the uninitiated. The old-time systematist would undoubtedly have regarded them as distinct species. MUTATION 67 De Vries* experiments and observations have been repeated on a large scale and extended, notably by MacDougal in the New York Botanical Gardens and by Shull at the Carnegie Institution for Ex- perimental Evolution, Cold Spring Harbor, Long Island, and his conclusions have been confirmed in all essential points. The mutability of 0. lamarcki- ana is as unmistakable and as diverse in America as it is in Holland. A parallel case of a plant caught in the act of giving rise to mutations is that of the roadside weed Lychnis, reported by Shull, and the phenomenon is probably by no means as unusual as is generally believed. The chief reason why such definite examples of mu- tation are so infrequently noted and recorded is because the attention of the investigator has generally been directed, not to them, but to gradual fluctuating variations which, according to Darwin's conception, furnish the material for the operation of natural selection. Mutations are doubtless much more com- mon than has been generally supposed, and it is likely that they will receive more attention in the future than they have in the past. De Vries rather pointedly says: "The theory of mutations is a starting-point for direct investigation, while the general belief in slow changes has held back science from such in- vestigations during half a century." 8. SOME MUTATIONS AMONG ANIMALS In 1791 a Massachusetts farmer, by name Seth Wright, found in his flock of sheep a male lamb with 68 GENETICS long, sagging back and short, bent legs resembling somewhat a German dachshund. With unusual foresight he carefully brought up this strange lamb because it was an animal that could not jump fences. It occurred to this hard-headed Yankee that it would be much easier to get together a flock of short, bow- legged sheep, unable to negotiate anything but a low hurdle, than to labor hard at building high fences. So it came about that this mutating lamb, in the hands of a man who appreciated labor-saving devices, be- came the ancestor of the Ancon breed of sheep. Later on this breed gave place in public favor to an- other mutant, the Merino, which produces a superior grade of wool. Hornless cattle suffer fewer injuries from one an- other than horned cattle. It has consequently be- come quite a general practice among farmers to "dehorn" their stock surgically. It is an obvious ad- vantage to have cattle born hornless, and many breeds having this character are now established. In 1889 a mutant among horned stock appeared at Atchison, Kansas, in the form of a hornless Hereford. From this mutant has descended the well-established race of polled Hereford cattle, constituting a bovine aristoc- racy with registry books and blue blood all their own. Taillessness in cats, dogs and poultry, as well as hairlessness in cattle, dogs, mice and horses, are further instances of mutations. Davenport, 1 writing of his experiments with poultry, 1 Davenport, C. B., 1909. " Inheritance of Characteristics in Domestic Fowl." Carnegie Institution of Washington, Publication No. 121. MUTATION 69 says: "During the past four years I have handled and described over 10,000 poultry of known ancestry. Of striking new characters I have observed many, some incompatible with normal existence; others in no way unfitting the individual for continued life. In the egg unhatched I have obtained Siamese twins, pug jaws, and chicks with thigh bones absent. There have been reared chicks with toes grown together by a web, without toenails or with two toenails to a toe ; with five, six, seven, or three toes ; with one wing or both lacking ; with two pairs of spurs ; with- out oil-gland or tail; with neck devoid of feathers; with cerebral hernia and a great crest; with feather shaft recurved, with barbs twisted and dichoto- mously branched or lacking altogether. Of comb alone I have a score of forms. All of these characters have been offered to me without the least effort or conscious selection on my part, and each appeared in the first generation as well-developed peculiarities, and in so far as their inheritance was witnessed, each refused to blend when mated with a dissimilar form." Bateson (1894), in his "Materials for the Study of Variation," gives a detailed list of 886 cases of "dis- continuous variations" among animals, many of which doubtless belong to the category of mutations, al- though several must be placed in the non-inherit- able class of "freaks." 9. POSSIBLE EXPLANATIONS OF MUTATION It is apparent that the causes of mutations, since they occur regardless of the environment, are prob- 70 GENETICS ably of an intrinsic or germinal nature. Evening primroses display the same mutants whether in Holland or America, in a wild state or under culti- vation. Mutations, like poets, are born, not made. It has been suggested by Standfuss that species may go through the same kind of a life-cycle that in- dividuals do, only taking infinitely more time to do it. As shown in Figure 32, they are born of other species and enter the prodigious growth period of infancy and youth, both of which are PIG. 32. Diagram of the relation of repro- characterized by duction to the life-cycle. in A much fluctuation. With maturity they gradually become comparatively stable until the reproductive period is reached, when they throw off their progeny, as on a tangent. They finally pass into the excessively differentiated period of old age, from which there is no recall, although they approach in many features the infantile condi- tion, and end in death or extinction. This cycle is repeatedly illustrated by phylogenetic lines of fossil forms which have long since become extinct. Beecher has pointed out ^.that, in paleontological times just before they became extinct, species often underwent extreme specialization in the form of MUTATION 71 fantastic shapes, an excessive number of spines or elaborate sculpturings on the shells as seen among the ammonites, belemnites, and trilobites, or of gigantic size as in the dinosaurs, plesiosaurs, and theromorphs. All of these facts indicate a species- cycle in which these abnormal features were the un- mistakable signs of old age. The reproductive period of a species when mutants are being thrown off, as of an individual, may ex- tend over a considerable period of the whole cycle, or it may be confined to a relatively small segment. It is possible that in the evening primrose de Vries may have caught a plant passing through the crucial period of species-reproduction. Another reason why so few mutations have as yet been seen, is because the majority of organisms are not, during the short span of human observation, in the reproductive part of their cycles. When it is remembered that accurate observation with this object in view has extended over only a brief period, insignificant in comparison with the vast geologic stretches of time concerned in species-building, the marvel is that so much, rather than that so little, has been seen. A further suggestion in connection with the pos- sible sources of mutation is that mutations may be the results of hybridization, appearing as Mendelian recessives after crossing. As a matter of fact, Spren- gel's Chelidonium ladniatum, already cited, when crossed with Chelidonium majus, behaves according to such an expectation. This phase of the question, 72 GENETICS however, may be more suitably considered later after what is meant by a "Mendelian recessive" has been made clear. It is extremely doubtful, however, whether any recombination of parental characters in a hybrid may properly be called a mutation, since no character strictly new is thus produced. 10. A SUMMARY OF THE MUTATION THEORY The main features of the mutation theory of de Vries may be indicated as follows : a. New species arise abruptly regardless of environ- ment without transitional forms, and at present they are not known to arise in any other way. b. New forms arise as unusual deviations from the parent form, which itself remains unchanged, al- though it may repeatedly give rise to similar devia- tions. c. New mutations are, from the first, constant, that is, they produce their like. They do not become gradually established as the result of natural selection. d. Among mutations there may occur forms characterized by the addition of something new, progressive elementary species, as well as forms lacking something present in the parental type, regressive varieties. e. The same mutation may arise simultaneously in many individuals instead of as a single " sport." /. Mutations do not vary around an arithmetical mean with respect to the parent form, as is the case MUTATION 73 with fluctuating variations, but they fluctuate around a new average of their own, thus forming a discon- tinuous series with the parent form. g. Mutations may occur in all directions, that is, they are not necessarily definite or orthogenetic. h. Mutations probably appear periodically. i. Every mutation means a possible doubling of the species. j. Useless or insignificant fluctuating variations are not necessarily the material from which natural selec- tion must sift out new species. The bearing of the whole matter of mutation upon heredity lies in the fact that, contrary to Darwin's belief, it is apparently mutations, and not fluctuations, that make up heritable variations. If this supposition proves to be true, mutations furnish the essential material in the study of heredity. Consequently, whatever knowledge we may gain of them has a direct relation to the entire problem of genetics. CHAPTER V THE INHERITANCE OF ACQUIRED CHARACTERS 1. SUMMARY OF PRECEDING CHAPTERS HEREDITARY resemblance is due to the derivation of offspring from the same stock as the parent, and suc- cessive generations, therefore, are simply periodic expressions of the same continuous stream of germ- plasm. Perfect inheritance, or uniformity of generations, does not exist, since variations always occur in suc- cessive generations. It is upon these variations that evolution depends. Without them there would be no change of type and consequently no possibility of evolutionary advance. Some variations are fluctuating or continuous in character and may be detected and analyzed by statistical methods, while others are mutations, or discontinuous variations, representing qualitative differences which do not lend themselves readily to statistical analysis. Mutations are more common than was formerly believed, and since they are germinal rather than somatic in character, they play an important r61e in heredity. 74 INHERITANCE OF ACQUIRED CHARACTERS 75 2. THE BEARING OF THIS CHAPTER UPON GENETICS Only those variations which reappear in succeeding generations have to do with heredity. Hence it be- comes important to inquire as to what kind of varia- tions actually reappear. Can variations that are not inborn, but which are acquired during the lifetime of the individual, be inherited ? Does the experience of the parent become a direct part of the child's heritage, or can the environment of the one enter in any way into the heredity of the other ? Can changes wrought in the somatoplasm be so impressed upon the germ- plasm as to change it in such a way that it, in turn, will give rise to similarly modified somatoplasm in the next generation? Can nurture as well as nature be transmitted ? In answering these questions we are of course con- cerned solely with biological inheritance and not at all with those extra-biological accumulations in the way of arts, literature, tradition, invention, and the like which constitute civilization and which make us the "heu*s of the ages." Such benefits are entailed upon us much in the same way as property is "in- herited," but they form no part of the personal bio- logical heritage into which we are now inquiring. 3. THE IMPORTANCE OF THE QUESTION This inquiry concerning the inheritance of acquired characters, which Professor Brooks has called "the interminable question," is not simply an academic matter. Its solution is of vital importance from 76 GENETICS several viewpoints. For breeders, who are trying to maintain or improve particular strains of animals or plants; for physicians, who, in fighting disease, are honestly seeking to substitute an ounce of prevention for a pound of cure; for sociologists and philanthro- pists, who have at heart the permanent bettering of human conditions ; for educators, who cherish hopes that their life-work of unfolding the youthful mind may prove cumulative and lasting rather than tran- sitory; for religious workers, who want their faith strengthened that conquests in character-building may outreach the individual and so enrich the race ; for parents, who entertain hopes that their own efforts may give their children a better biological start in life, for all these and many more, it is important to know the answer to the question : Can acquired characters be inherited ? 4. AN HISTORICAL SKETCH OF OPINION That the personal accumulations of a lifetime are heritable was generally believed all through the credulous ages. A century ago Lamarck made this idea the corner-stone of his theory of evolution. It was all very simple. The reason evolution occurs in nature is because individual acquirements are being continually added to the onflowing stream of living forms. This cumulation of characters indeed is evolution. How else can the present stage of adaptation of organisms to their several niches in nature be explained save by seeing in it the final results of generations of gradually inherited adaptations ? INHERITANCE OF ACQUIRED CHARACTERS 77 Darwin also believed in the inheritance of acquired characters, although he differed from Lamarck with respect to how such characters are acquired. Francis Galton in 1875 was one of the first to ex- press skepticism regarding this generally accepted belief but the man who, in a masterly manner, fo- cused the growing doubt, and who did more than any other to inspire thought and investigation upon the subject, was August Weismann, for nearly fifty years professor in the University of Freiburg in Baden. Weismann made the issue so clear that the heritability of acquired characters became the parting of the ways which divided biologists into the two camps of Neo-Lamarckians who affirm, and Neo- Darwinians who deny, such inheritance. If the question could be decided by a vote or by an expres- sion of opinion, the result would be doubtful, since each column contains the names of men whose scien- tific accomplishments entitle them to a respectful hearing. Geneticists and embryologists, represent- ing the two lines of study which furnish the most immediate approach to this problem, are well-nigh agreed, however, that acquired characters are not inherited. But just what are the facts of the case ? 5. CONFUSION IN DEFINITIONS The source of much of the lack of agreement in this controversy lies in the definition of what con- stitutes an "acquired character." One is reminded of the two knights who fought so bitterly over the 78 GENETICS color of a shield, one maintaining that it was red, the other that it was black. So they hacked away at each other, as all good knights should do in the defense of the truth, until they both fell down dead beside the shield which was black on one side and red on the other. Of course actual characters are never inherited, but only the determiners or potentialities which regulate the way in which the organism reacts to its environ- ment or training with respect to the characters in question. Reid has pointed out that in one sense every adult character is "acquired" because it has no expression at first. For instance, there is no beard on the face of a male infant, but it will presum- ably be "acquired" later on in the life-cycle. It is plain that every new character which repre- sents a forward evolutionary step must have been "acquired," or added, sometime and somewhere, else it would not be present, as there is evidence that it is. Perhaps the question, as Montgomery has suggested, ought to be changed to read : " What kinds of acquired characters are inherited ? " It is obvious that dis- cussion is futile until a common denominator in the shape of a definition of acquired characters shall be accepted. 6. WEISMANN'S CONCEPTION OF ACQUIRED CHARACTERS Weismann defines an acquired character as any somatic modification that does not have its origin in the germplasm. INHERITANCE OF ACQUIRED CHARACTERS 79 Of course those somatic modifications which are phases of the developing individual, such as the acquisition of a deeper voice at puberty or the sub- stitution of the permanent dentition for the milk- teeth, are somatic variations which have their rise and control in the germplasm and consequently cannot properly be included under the head of ac- quired characters. Examples of acquired characters in the Weis- mannian sense are mutilations, the effects of environ- ment, the results of function as in the use or disuse of certain organs, and such diseases as may be due either to invading bacteria or to the neglect or abuse of the bodily mechanism. 7. THE DISTINCTION BETWEEN GERMINAL AND SOMATIC CHARACTERS Redfield has recently thrown light on the classi- fication of the characters which make up the indi- vidual by quoting the familiar lines : " Some are born great, Some achieve greatness, Some have greatness thrust upon them." "Born" characters are constitutional, having their origin in the germplasm itself. They are never Weismannian acquired characters and may be illustrated by eye-color, mental disposition, or facial features. Lightning calculators and musical prodi- gies may have their gifts developed and enlarged, 80 GENETICS but the fact that their talent is nevertheless an unmistakable gift, and not an acquisition, remains true. "Achieved" characters are functional and are gained by exercise. Many things are achieved, however, which are not acquired characters, as, for instance, wealth, reputation, or an education. Not any of these are biological characters, and therefore we are not concerned with them in this connection, although in the case of education it should be noticed that the mental exercise necessary to bring about a trained mind, if not the subject matter of the edu- cation itself, is distinctly an acquired character of the "achieved" type. "Thrust" characters are the results of environ- ment. They are acquired without functional activity on the part of the organism and usually in spite of anything the organism can do to prevent. Some- times these characters are thrust upon the individual before birth, as in the case of blindness caused by parental gonorrhoea or tuberculosis arising from uterine infection, in which case they are termed con- genital characters. Congenital or prenatal characters, however, are in no way the same as germinal characters, for they fall just as truly into the category of acquired variations as do those which make their appearance in later life. 8. WHAT VARIATIONS REAPPEAR? Returning now to Montgomery's question, "What kinds of acquired characters are inherited?" it INHERITANCE OF ACQUIRED CHARACTERS 81 is apparent that only the "born" ones can be, which have their roots in the germplasm whence the new individual arises, and that "achievements" and "thrusts," in order to reappear in the succeeding gen- eration, can do so only by first becoming incorporated in the germplasm. Any character that is not acquired must have been present in the germplasm from which the organism arose, as there is no transfer of characters between organisms except through the germ-cells. Thus it is evident that the only inherited acquisitions are those which, either primarily or secondarily, bring about variation in the germplasm. Such temporary acquisi- tions as a coat of tan or a display of freckles do not impress the germplasm, for when the cause that in- cites their appearance is removed, they soon vanish. 9. WHAT MAY CAUSE GERMPLASM TO VARY OR TO ACQUIRE NEW CHARACTERS? The causes which bring about changes in the germ- plasm may be either internal or external. Of possible internal causes may be mentioned first the "amphimixis" of Weismann, that is, the mixture of two nearly related strains of germplasm in sexual reproduction within a species, or secondly, the mixture of two more remotely related strains resulting in hy- bridization. In either case the strain of germplasm undergoes a shake-up that may result at least in new combinations of characters, if not in the production of entirely new characters. This recombination of 82 GENETICS characters in amphimixis and hybridization will re- ceive further attention in a later chapter. The fact that successive parthenogenetic genera- tions, in which amphimixis does not of course occur, may show a larger degree of variability than sexually produced generations, indicates that amphimixis in itself is by no means sufficient to account for all kinds of variations. , The abrupt way, for instance, in which mutations appear in apparent independence of external influences suggests that there may be some internal factor, as yet unknown, acting directly through the germplasm, regardless of external causes. The assumption of an unknown factor does not necessarily imply a return to "vitalism," which is so elusive of experimental test and hence so unsatisfac- tory to the scientific mind, nor does it admit, simply because this factor is at present an unknown quantity, that it is consequently doomed to remain so. It is easily conceivable that the external factors acting upon the germplasm may be grouped into two alternative classes : first, external factors that act upon the somatoplasm and through the agency of the somatoplasm affect the gennplasm ; and second, those that act directly upon the germplasm without neces- sarily at the same time influencing the somatoplasm. The first category, that of somatic modifications which leave their impress upon the germplasm, in- cludes true acquired characters according to our definition, while the second, which includes cases of the direct influence of external stimuli upon the INHERITANCE OF ACQUIRED CHARACTERS 83 germplasm, regardless of any simultaneous modifica- tion of the somatoplasm, must be excluded as irrele- vant to a. discussion of the heritability of acquired characters in the Weismannian sense, since they are not somatic modifications at all. Many instances of direct influence of external stimuli upon germplasm are known in biological literature, and these have led to some of the misunder- standings concerning the "interminable question" of the inheritance of acquired characters. MacDougal, for example, was able by injecting certain salts into the carpels of plants to stimulate the germplasm of the forming seeds so directly that a progeny of modified character was produced which, in succeeding generations, apparently bred true to the newly induced character. This is an instance of what has been termed ** parallel induction," where somato- plasm and germplasm are affected together by an ex- ternal factor, as opposed to "somatic; induction" or Weismannian acquired characters, in which the germ- plasm is secondarily influenced through, or by the agency of, the somatoplasm. Sitkowski fed the cater- pillars of the moth Tineola biselliella with an aniline dye (Sudan red III) , obtaining therefrom, instead of the normal whitish ones, moths that laid colored eggs, and these in turn hatched into caterpillars still tinged with the color of the red dye. Riddle, with guinea- pigs, and Gage, with poultry, obtained quite similar results. This apparent case of parallel induction, however, is not a matter of inheritance at all, but of animals who got their red color while they were eggs within the mother's body. 84 GENETICS 10. WEISMANN'S REASONS FOR DOUBTING THE INHER- ITANCE OF ACQUIRED CHARACTERS Weismann's reasons for questioning the popularly accepted view that acquired characters are inherited may be briefly stated as follows : First, there is no known mechanism whereby somatic characters may be transferred to the germ- cells. Second, the evidence that such a transfer actually does occur is inconclusive and unsatisfactory. Third, the theory of the continuity of the germ- plasm is sufficient to account for the facts of heredity without assuming the inheritance of acquired somatic characters. Let us examine these three statements a little more closely. 11. No KNOWN MECHANISM FOR IMPRESSING THE GERMPLASM WITH SOMATIC CHARACTERS The somatoplasm is something that has traveled out from the original fundamental germplasm along the paths of differentiation and elaboration. The more complex the body cells become, that is, the more successive modifications they undergo, the more difficult it is for these somatic cells to return to their original primitive estate. In many lower forms of life where cell elaboration is not so great, a part lost by amputation is often regenerated, but this process is not possible in higher INHERITANCE OF ACQUIRED CHARACTERS 85 forms where the parts represent cell complexes too hopelessly differentiated to begin anew the unfolding sequences of their elaboration. This difficulty was a very real one in the mind of that famous nocturnal inquirer Nicodemus when he asked: "Ho wean a man be born when he is old ? Can he enter a second time into his mother's womb and be born ? " Not only the development of the race which we call evolution, but also the determination of the individual in heredity, is a chain of onward-moving sequences like the succession of events in history. It is hard to see how recent events can influence pre- ceding events. It is hard to see how the water that has gone over the dam can return and affect the flow of the river upstream in any direct way. It is like- wise hard to see how differentiated somatoplasm, which represents the end stage of a successive series of modifications, can make any definite impress upon the original germplasmal sources from which it arose. Darwin felt this difficulty and presented with apolo- gies his provisional hypothesis of pangenesis in which he assumed that every bodily part sends contributions to the germ-cells in the form of "gemmules." These gemmules, or hypothetical somatic delegates, then reconstruct in the germ-cells the characters of the en- tire body, including acquired modifications as well as all others, and thus there is no reason why acquired characters cannot readily be transmitted. Unfortu- nately there is no tangible basis in fact for this de- lightfully simple explanation to rest upon. It is a theory assuming that all parental somatic cells take 86 GENETICS part in the formation of the new individual, hence it was called "pangenesis," or origin from all. Nothing we have subsequently learned of minute cell structure favors this hypothesis, while many facts go quite against it. Moreover, it is directly opposed to the theory of the continuity of germplasm so convincingly set forth later on by Weismann. Darwin indeed advanced it only in the most tenta- tive way, being entirely ready to see it abandoned at any time for something better. It at least per- formed one valuable service to science, namely, that of demonstrating how far investigators were from an adequate conception of any means by which somatic modifications might become incorporated in the germ-cells. We must acknowledge, however, with Lloyd Morgan that the fact that a mechanism for the trans- fer of somatic characters to the germ-cells has not been discovered, is not proof that such a mechanism does not exist. It may simply be beyond our present powers of penetration. 12. EVIDENCE FOR TRANSMISSION OF ACQUIRED CHARACTERS INCONCLUSIVE The evidence for the inheritance of acquired characters was, for a long time, taken for granted. This theory was the most obvious explanation of many facts and so was accepted without question. An obvious interpretation, however, is not always the correct one. The sun appears to go around the earth, but astronomers assure us that it does not. INHERITANCE OF ACQUIRED CHARACTERS 87 When Weismann began to sift the evidence for the inheritance of acquired characters, he found that it was largely based upon opinion rather than fact, much like the popular belief with regard to prenatal influences and birthmarks, or the causation of warts by handling toads. The supposed evidence for the inheritance of ac- quired characters falls chiefly into four categories : a. Mutilations; 6. Environmental effects; c. The effects of use or disuse ; d. The transmission of disease. a. Mutilations It is fortunate that the sons of warriors do not inherit their fathers' honorable scars of battle, else we would now be a race of cripples. The feet of Chinese women of certain classes have for centuries been mutilated into deformity by band- aging, without the mutilation in any way becoming an inherited character. The same result is also true of circumcision, a mutilation practised from ancient times by the Jews and certain other Eastern peoples. The progressive degeneration or crippling of the little toe in man has been explained as the inheritance of the cramping effect of shoes upon generations of shoe wearers, but, as Wiedersheim has pointed out, the fact that Egyptian mummies show the same crippling of the little toe is unfavorable to this hypothesis, for no ancient Egyptian could 88 GENETICS ever be accused of wearing shoes or of having had shoe- wearing ancestors. Sheep and horses with docked tails as well as dogs with trimmed ears never produce young having the parental deformity. Weismann's classic experiment with mice, an experiment subsequently confirmed by others, is additional negative evidence upon this same point. What Weismann did was to breed mice whose tails had been cut off short at birth. He continued this decaudalization through twenty-two generations with absolutely no effect upon the tail-length of the new-born mice. One may see in the catacombs of the Zoologisches Institut at Freiburg, filed carefully away on shelves, as a " document," long rows of labeled bottles containing the fifteen hundred and ninety-two martyrs to science which made up the twenty-two generations of mice in this famous experiment. Conklin has hit the nail upon the head with respect to mutilations by saying: "Wooden legs are not in- herited, but wooden heads are." b. Environmental Effects Trees deformed by prevailing winds, like the willows that line the canals in Belgium and Holland, or storm-crippled trees along the exposed seacoast are not known to produce a modified progeny when their adverse environmental conditions are removed. Similarly, the persistent sunburn of Eng- lishmen long resident in India does not reappear in their children born in England. INHERITANCE OF ACQUIRED CHARACTERS 89 Sumner kept mice in a constant but abnormally high temperature of 26 C. with the result that the ears, tail, and feet grew noticeably larger than in control animals kept in ordinary lower temperatures, while at the same time the general hairiness of the body decreased. It should be remembered, however, that mice are mammals which pass through an ex- tended uterine existence, so that it is easy to see how the offspring in this case were subjected to the same excessive temperature as the parents for a period sufficient to amply account for their subsequent variation when removed to a normal environ- ment. Zederbaur finds that the wayside weed Capsella, which in the course of many years has gradually crept along the roadside up into an Alpine habitat and there "acquired" Alpine characters, upon being transplanted to the lowlands retains its Alpine modifications. Although this case has been cited as an authentic instance of the inheritance of ac- quired characters, is it not possible that the conquest of the Alps by Capsella has been due, in the course of time, not to the inheritance of acquired characters at all, but to a gradual natural selection of just those germinal variations which best fitted it to cope with Alpine conditions until, finally, a strain of germplasm producing somatoplasm suitable to Alpine conditions has been isolated in the form of an elementary species derived from the original type ? If this is what has happened, of course such germplasm would give rise to Alpine plants whether individual plants grew 90 GENETICS to maturity near the snow-line or in the warm valleys at a lower altitude. Marie von Chauvin was able, by decreasing the amount of water in an aquarium, to transform the gill-breathing salamander Axoloil into the land form, Amblystoma, which in its adult form has no gills, but breathes by means of lungs. Both of these forms are sexually mature, reproducing their like, and had long been recognized by systematists as distinct species. More recently Kammerer, by similarly reducing the water supply, succeeded in transforming Sala- mandra maculosa, a salamander that normally pro- duces about seventy eggs which, when hatched in water, become gill-breathing tadpoles, into a sala- mander producing only two to seven young which are born alive without gills and are able to live out of water entirely, in damp situations. These land- adapted offspring, moreover, when supplied with abundant water, produce in turn tadpoles which spend days only, instead of months, in the water under- going their metamorphosis, thus showing an appar- ent inheritance of an acquired character. It should be pointed out, however, that in these cases the gill-breathing forms in each instance rep- resent a case of arrested development. Axoloil is simply a larval form of Amblystoma that, under nor- mal conditions of an abundant water environment and high temperature, gets no further in its meta- morphosis than the tadpole stage, when it produces eggs and sperms and finishes its life story. A change INHERITANCE OF ACQUIRED CHARACTERS 91 in environment simply permits the life-cycle to go on further. Changing from gill-breathing to lung- breathing is not, therefore, an acquired character, but a purely germinal character that may be either blocked or released by changing conditions in the environment. c. The Effects of Use or Disuse The callosities on the end of a violinist's left-hand fingers are acquired by use, but they are not inherited. There are callosities on the knees of the wart-hog, Phacechcerus, which are also apparently the result of use, for these animals kneel as they root for a living in the African forests, and have done so for untold generations. It has been noticed that young wart- hogs as soon as they are born possess the callosities, so that this instance looks like one of inheritance of a character acquired through use or exercise. The skin on the soles of human feet is thicker than the skin elsewhere, and by use it becomes still thicker. This is apparently another instance of the same sort. The writer has observed, however, that a cross sec- tion through the foot of a "mud puppy," Necturus maculatus, shows a much thickened sole. Necturus, it should be noted, is a very primitive salamander living always under water and never using the soles of its feet in any way to bear its weight, nor is it reasonable to suppose that it ever had any ancestors who did so, for the hands and feet of the Amphibia are the most primitive and ancient hands and feet to be found in the animal kingdom without any known 92 GENETICS ancestral types. The thickening of the skin on the sole of the mud puppy's feet must be due, therefore, to germinal determiners and is in no way an acquisi- tion through use. The same may also be true of the wart-hog's knees and of human soles. The strong arm, the skilled hand, and the trained ear are not inherited. They have always to be reacquired in each succeeding generation just as surely as the ability to walk, or to read and write. Herbert Spencer has defined instinct as "inherited habit." But surely those instincts which determine a single isolated action during the lifetime of the individual, such as the spinning of a peculiar cocoon, cannot be the result of habit, since habits are formed only through repeated action. If, then, some in- stincts require a different explanation from that of "inherited habit," may it not be likely that all in- stincts do ? Dr. Hodge, who succeeded in hatching tame quail chicks out of "wild" eggs, asks the perti- nent question: "How can a fear hatch out of an egg?" The habit of wildness, particularly with precocial chicks like quails, may, under an inciting environment, be very soon established, but it is diffi- cult to see how caution, gained by the experience of the parents, can find its way into the fertilized egg. d. Disease Transmission Many diseases, like tuberculosis, have their im- mediate cause in invading pathogenic bacteria. Bacteria themselves cannot be inherited for the reason that it is not possible for them to become an INHERITANCE OF ACQUIRED CHARACTERS 93 integral part of the fertilized egg and thus cross the "hereditary bridge" which joins two generations. A general predisposition to bacterial disease, that is, a lack of resistance to bacterial invasion due to de- fectiveness in physical or physiological equipment, may be present as a combination of characters in the germplasm, or an individual, as the result of disease, may "acquire" a generally weakened germ- plasm and so produce a progeny exhibiting general liability to disease ; but it is doubtful if such a con- dition can properly be termed the inheritance of an acquired character, since the particular definite disease in question is not demonstrably heritable. When alcoholism "runs in a family," its reappear- ance in the son is probably due to the fact that he is derived from the same weak strain of germplasm as his father. The fact that the father succumbed to the alcohol habit is not the determining cause of drunkenness in the son. The same thing that caused the father to become an alcoholic, namely, weak germplasm, and not the resulting drunkenness in the parent, is the causal factor for alcoholism in the son. At the same time it is entirely probable that hered- itary alcoholism may in some cases arise through "parallel induction," that is to say, acquired alco- holism may end in the simultaneous poisoning and consequent modification of both the somatoplasm and germplasm of the parent, with the result that the germplasm has less resistance to alcoholism in a suc- ceeding generation. The offspring are consequently more likely to succumb to the disease. This, how- 94 GENETICS ever, is not the inheritance of an acquired character or of a definite somatic modification. When a man of the present generation has rheu- matic gout, it is a severe stretch both of patriotism and of the powers of heredity to trace the origin of the affliction back to a revolutionary ancestor who acquired sciatic rheumatism by sleeping on the ground at Valley Forge, yet this is quite as direct as many alleged instances of the inheritance of disease. In the majority of instances, apparent cases of disease inheritance are merely instances of reinfec- tion. This reinfection of the offspring may occur very early in embryonic life, or it may happen after birth, provided the offspring are exposed to the same environment as that in which the parent acquired the disease, but in any case reinfection is not heredity. 13. THE GERMPLASM THEORY SUFFICIENT TO AC- COUNT FOR THE FACTS OF HEREDITY Weismann holds that the theory of the continuity of the germplasm, already considered in a previous chapter, is sufficient in itself to account for the facts of heredity. Hence it is quite unnecessary to fall back upon the inheritance of acquired characters as an ex- planation, since this theory is at least difficult, if not impossible, of satisfactory proof. To prove the inheritance of acquired characters, according to Weismann three things are necessary : first, a particular somatic character must be called forth by a known external cause ; second, it must be something new or different from what was already INHERITANCE OF ACQUIRED CHARACTERS 95 exhibited before, and not be simply the reawakening of a latent germinal character; and third, the same particular character must reappear in succeeding generations in the absence of the original external cause which brought the character in question forth. As yet these conditions have not been convincingly met in the evidence which has been brought forward in support of the inheritance of acquired characters. 14. THE OPPOSITION TO WEISMANN The opponents of Weismann point out, as a weak place in his argument, the assumption that the germ- plasm is so insulated from the somatoplasm as not to be influenced by it. Weismann assumes, of course, that the germplasm is isolated from the somatoplasm very early in the development of the fertilized egg into an individual, and that when once isolated it there- after takes no active part in, nor is in any way affected by, the vicissitudes through which the somatoplasm, or the body itself, passes. The somatoplasm is thus merely a carrier of the germplasm and unable to affect the character of it any more than a rubber hot- water bag, although capable of assuming a variety of shapes, can affect the character of the water within it. In opposition to this view it is urged that every organism is a physiological as well as a morphological unity, and that cells entirely insulated within such a unity would be a physiological miracle. There is abundant evidence that germ-cells, or rather the sexual organs producing the germ-cells, do 96 GENETICS affect the somatoplasm under particular conditions, as, for example, in cases of castration when those somatic features called "secondary sexual charac- ters" undergo profound modification. If the germplasm thus exercises a constant influ- ence on the somatoplasm, why, it seems legitimate to ask, may not the reverse be true and acquired somatic characters leave their impress upon the germ-cells ? 15. CONCLUSION But even granting the reverse to be true, that is, that the somatoplasm affects the germ-cells, the in- heritance of acquired characters is by no means thereby established. In order to do this, the precise acquired character in question, which indirectly exercised its influence upon the germ, must be redeveloped, and, although the germplasm might conceivably receive an influ- ence from the somatoplasm and be affected by it in a general way, it is a different matter entirely to develop anew the verisimilitude of the character itself which is supposed to have been acquired. It will be seen in subsequent pages, under the dis- cussion of data furnished by experimental breeding, that the weight of probability is decidedly against the time-honored belief in the inheritance of acquired characters. CHAPTER VI THE PURE LINE 1. THE UNIT CHAKACTER METHOD OF ATTACK IN reducing any body of facts to a science, it is first necessary to determine the underlying units out of which the facts are made up. Chemistry was alchemy until the chemical ele- ments were identified and isolated. Histology was terra obscura until the cell theory brought forward "cells" as the units of tissues. In the same way there could be no science of genetics until the con- ception was developed that the individual is a bundle of unit characters rather than a unit in itself. So it has come about that we now speak of inheritance as applied to unit characters rather than to individuals as a whole. Incidentally the fact that an organism is a com- bination of many units makes it easy to account for the wide diversity of forms found in nature, since the addition of a single unit greatly increases the pos- sible combinations in successive generations. Thus if three unit characters, A, B, and C, are present in each parent, for example, there would be six possible double combinations in the offspring, namely, AA, AB, AC, BB, BC, and CC. If now a H 97 98 . GENETICS fourth unit D is added by one parent, there would be not only the original six double combinations, but in addition to these, AD, BD, and CD, that is, as many more as there are unit characters with which the new one may combine. Obviously, when individuals are made up of very many unit characters, as, for instance, a thousand, the addition of one new unit character will increase the possible double combinations a thousand fold. 2. GALTON'S LAW OF REGRESSION Galton was one of the first 1 to attempt to express mathematically the relationship between parents and offspring by means of treating statistically a single unit character. According to Galton, a mathe- matical expression of the relationship between two generations should serve as a corner-stone of heredity. What Galton did was to take human stature as a unit character in comparing 204 English parents and their 928 adult offspring, because human stature is not complicated by environmental influences and is, consequently, a purely hereditary matter. Since female height is normally less than male height, the two were reduced, for purposes of comparison, to a common male denominator by multiplying each female height by 1.08, which is the average amount that the male exceeds the female in height. There are always two parents con- cerned in the sexual production of every offspring, 1 " Hereditary Genius," 1869. THE PURE LINE 99 therefore Gallon reckoned a "midparent" in each case, according to the formula 1.00 $ + 1.08 $ 2 in order to represent the double parental generation by a single number for the purpose of easy compari- son with the filial generation. 63 FIG. 33. Scheme to illustrate Gallon's law of regression. The circles represent graded groups of parental height while the arrowpoints in- dicate the average heights attained by the respective offspring. The offspring of undersized parents are taller, and of oversized parents are shorter than their respective parents. Based on data from Galton. The results of his measurements expressed in inches are shown in the following table, in which the 100 GENETICS offspring are, in each instance, arranged under their respective midparents. Midparental height . . 64.5 65.5 66.5 67.5 68.5 69.5 70.5 71.5 72.5 Average height of offspring . 65.8 66.7 67.2 67.6 68.S 68.9 69.5 69.9 72.2 The mean group for all the midparents, it will be seen, is 68.5, and the offspring of this group average 68.3. The table is expressed graphically in Figure 33 in which the circles connected by the diagonal line represent the graded parental heights, while the arrowpoints indicate the average heights of the off- spring in each group. In order to compare these two series of numbers more readily, they may be reduced to a common basis in which the mean class in each instance is made equal to 100, as follows : Midparental height . . 94 95.5 97 98.5 100 101.5 103 104.5 106 Average height of offspring . 96 97.5 98.5 99 100 101 101.5 102 105.5 The same series may be expressed in terms of amount of deviation from the mean or middle classes, as shown below. The deviation of each group in the series is marked by the signs + or according as the heights given are greater or less than 100. THE PURE LINE 101 Midparental height . . - 6 -4.5 -3 -1.5 + 1.5 + 3 + 4.5 + 6 Average height of offspring . -4 -2.5 -1.5 -1 + 1 + 1.5 + 2 + 5.5 Finally, the relation between the midparent and the average offspring may be expressed in fractional form by taking the average height of the offspring for the numerator and the height of the midparent for the denominator in each instance. The minus deviations are thus seen to be 4 8.5 ,1.5 1 6 4.5 3 1.5 which, added together, divided by four and reduced to a decimal, equal .60. Similarly, the plus deviations are 1 1.5 2 5.5 1.5 3 4.5 6 which reduce to .63. The average of the minus deviations (.60) and the plus deviations (.63) is nearly .62, or about two thirds. That is to say, the fraction f represents the amount of resemblance or "inheritance" between two generations, as determined by the foregoing series, while the remaining ^ is the measure of "regression" from the general type. This illustrates Galton's Law of Regression or the tendency in successive generations toward medioc- rity. The law may be stated as follows : 102 GENETICS Average parents tend to produce average children ; minus parents tend to produce minus children ; plus parents tend to produce plus children ; but the progeny of extreme parents, whether plus or minus, inherit the parental peculiarities in a less marked degree than the latter were manifested in the parents themselves. 3. THE IDEA OF THE PURE LINE It was Galton's law of regression that suggested to the Danish botanist Johannsen a possible means of controlling heredity. In his mind arose the ques- tion whether it would not be possible by continually breeding from plus parents, granting that plus par- ents produce plus offspring and making allowance for some regression to type, to shove over the off- spring more and more into the plus territory and so to establish a plus race. To test this hypothesis, Johannsen selected beans, Phaseolus, with which to experiment, since this group of plants is self-fertilizing, prolific, and easily measurable. Somewhat to his surprise, his beans refused to shove over as much as expected. That is, big beans did not yield principally big offspring, nor little beans little offspring, according to the ex- pectation, although they each produced offspring that varied in the manner of fluctuating variability around an average unlike the parental type. This gave Johannsen the idea that he was using mixed material, so he next isolated the progeny of single beans, which, being self-fertilized, each constituted unmistakably a single hereditary line. In this way THE PURE LINE 103 nineteen beans, now famous, became the known ancestors of Johannsen's original nineteen "pure lines," a further study of which has led the way to some of the most brilliant biological discoveries of recent years. A pure line has been defined by Johannsen as " the descendants from a single homozygous organism exclusively propagating by self-fertilization," and more briefly by Jennings as "all the progeny of a single self -fertilized individual." 4. JOHANNSEN'S NINETEEN BEANS It was found by Johannsen that the progeny of each of these pure lines of beans varied around its own mean, which was different in each of the nine- teen instances. When, however, extremes from any pure line series were selected and bred from, the prog- eny, instead of showing two thirds inheritance and one third regression with respect to the extremeness of a particular character, as Galton found was true in the case of human stature, showed no inheritance and complete regression away from the extreme condition of the parent bean back to the type for the entire pure line in question. That is, selection within a pure line is absolutely without effect in modifying a particular character in the offspring of the line in question. This is illustrated in Figure 34 in which the results of selecting for size in the year 1902 is shown for four pure lines only. The average for each pure line is given at the top of its column. When, for example, 104 GENETICS beans weighing 60 eg. were selected from pure lines II, VTI, and XV, the average weights of their progeny were 56.5, 48.2, and 45.0 eg. respectively, which in each instance is nearer to the average for the pure line than to the weight of the parental seed. JO to W 40 TO Pr U. mmfer 51 XZ 2M Pio. 34. The result of selection in four pure lines of beans. The verti- cal columns, representing the average progeny from different sized parents all derived from the same pure line, contain groups nearer alike than the horizontal columns, representing progeny from the same sized parents, but different pure lines. All the numbers indicate centigrams. Data from Johannsen. It will be seen at once that the averages in the vertical columns are nearer alike than the averages in the horizontal columns. In other words, the beans bred true to their pure line rather than to their fluctuating parent. As a further example of this law, take the result THE PURE LINE 105 of selection for six years in pure line I as shown in the accompanying table and in Figure 35. MEAN WEIGHT OF SELECTED PARENT SEED MEAN WEIGHT OP OFFSPRING TT a V-o. i Minus Plus From Minus Parent From Plus Parent 1902 .... 60 70 63.15 64.85 1903 .... 55 80 75.19 70.88 1904 .... 50 87 54.59 56.68 1905 .... 43 73 63.55 63.64 1906 .... 46 84 74.38 73.00 1907 .... 56 81 69.07 67.66 It is evident, for instance, that in 1907 the smallest beans, weighing an average of 56 eg., gave an average progeny weighing 69.07 eg, while the largest ones for the same year, weighing an average of 81 eg., produced nearly the same average in their progeny as did the smallest beans, that is, 67.66 eg. Incidentally all the progeny from both large and small parents averaged notably less in 1904 than all the progeny from large and small parents in 1906, a result due to a "poor year" when certain factors of environment were unfavorable. Such unfavor- able conditions, however, are known to influence in no way the hereditary qualities of the beans. Thus it appears that, although the progeny of a pure line present plenty of variations of the fluctuating type, due probably to environmental differences in nutri- tion, moisture, etc., such variations are quite inef- GENETICS THE PURE LINE 107 fectual so far as inheritance is concerned, and it makes no difference whether the largest or the small- est beans within a pure line are selected from which to breed, the result will be the same, in that there is a complete return to mediocrity or type with no "in- heritance" of the parental modification. As a matter of fact in 1903, 1906 and 1907 the lighter parents gave a heavier progeny than the heavier parents. It will be seen at once that here is a discovery of far-reaching importance which may require us to reconstruct certain cherished ideas about the part played in the evolution of species, as well as in heredity, by natural selection. 5. CASES SIMILAR TO JOHANNSEN'S PURE LINES Although according to Johannsen pure lines are "the progeny of a single self-fertilized individual," it is plain that in at least three other possible cases something quite similar to "pure lines" may be obtained. First, when two similar organisms identical in their germinal determiners with regard to a particular character inbreed, their progeny will form a pure line so far as this particular character is concerned just as truly as two parents that are united in a single in- dividual produce a pure line progeny as the result of self-fertilization. Second, in cases of parthenogenesis, the progeny arising from a single female individual without the customary qualitative reduction of chromosomes that 108 GENETICS accompanies sexual reproduction, constitute a pure line or an unmixed strain. Third, in cases of asexual reproduction where the progeny are simply the result of continued fission of the original individual, a pure line may be said to continue from generation to generation. In the second and third categories it should be pointed out that the "pure line" is assured only so long as asexual reproduction continues. It is quite possible for an organism, heterozygotic in composi- tion, to continue to breed true or to produce an ap- parently pure line so long as asexual methods are employed. As soon as such an organism, however, changes to the sexual method of reproduction, seg- regation of characters may occur and different combinations result. 6. TOWER'S POTATO-BEETLES As an illustration of the effect of selection within pure lines of the first category may be mentioned a case given by Tower in his exhaustive experiments on the Colorado potato-beetle Leptinotarsa decem- lineata. Among the numerous cultures of this beetle which were under control, a considerable variation in color made its appearance. For con- venience in classification these variations were graded into arbitrary classes or graduated variants (see p. 52) ranging from dark to light. When a male and a female from the extreme class at the dark end of the series were allowed to breed together, their progeny were not dark, but fluctuated xir XT X W FIG. 36. Diagram showing the ineffectiveness of selection through twelve generations within a homozygous strain in the case of the Colorado potato-beetle (Leptinotarsa) . In each generation extreme dark speci- mens were selected as the parents of the succeeding generation but the progeny always swung back to the type. After Tower. 110 GENETICS in color around the original average of the entire series. This process of selecting each time an ex- treme pair of dark parents was continued for twelve generations, as shown in Figure 36, without in any way increasing the percentage of brunette potato beetles in the progeny. Thus in a pure line formed by the breeding of two individuals alike with respect to color, the selection of an extreme variant was quite without effect in modifying the color of the progeny. 7. JENNINGS' WORK ON PARAMECIUM An instance of the third category of pure lines is furnished by Jennings' remarkable work on the protozoan Paramecium, which was published in 1909. Jennings carried on his experiments quite independ- 206 200 FIG, 37. Eight pure races of Paramecium. The actual mean length of each race is given in micra below the corresponding outline. Magni- fied about 230 diameters. After Jennings. ently of Johannsen, but he nevertheless arrived at the same general conclusion, namely, that selection within a pure line is without effect. Jennings found that Paramecia differ from each THE PURE LINE 111 other in size, structure, physical character, and rate of multiplication as well as in the environmental conditions required for their existence and, further- more, that these differences, in an hereditary sense, are "as rigid as iron." With respect to the character of mean length he was able to isolate eight races, or pure lines, whose average size, drawn to scale, is shown in Figure 37. 256 <- MICRA -> 80 FIG. 38. Diagram of a single race (Z>) showing the variation in the size of the individuals. Magnified about 230 diameters. After Jennings. Each of these pure lines produced a progeny which exhibited a considerable range of fluctuating variation. The offspring of pure line D, for example, varied from 256 to 80 micra l in length with an aver- age of 176 micra, as shown in Figure 38, where samples of the different classes of variants in pure line D are arranged in a series. A single representative of each of the different classes of variants out of all of the eight pure lines bred by Jennings is shown in Figure 39. Each horizontal row represents a single race or 1 A micron is a millimeter. 112 GENETICS pure line, the average size of which is indicated by the sign -f . The mean length of the entire lot, as shown 155 FIG. 39. Diagram of the species Paramecium as made up of the eight different races shown in Figure 37. Each horizontal row represents a single race. The individual showing the mean size in each race is in- dicated by a cross placed above it. The mean for the entire lot is at the horizontal line. The magnification is about 24 diameters. After Jennings. by the vertical line, is 155 micra. The total number of individuals belonging to each size is not indicated, but in every horizontal line their number is more THE PURE LINE 113 numerous near the average for that line and less numerous at the extremes, thus forming the typical normal frequency polygons of fluctuating variability. The significant fact about these series is this, that extreme individuals selected from any pure line do not reproduce extreme sizes like themselves, but instead, a progeny varying according to the laws of chance around the average standard of the particular line from which it came. 8. PHENOTYPICAL AND GENOTYPICAL DISTINCTIONS From the foregoing it will be seen that the be- havior of an organism in heredity cannot always be determined by an inspection of its somatic char- acters alone. For example, six Paramecia, each 155 micra in length and apparently identical, could be selected from the six upper pure lines in Jennings' table given in Figure 39 which would produce six progenies definitely unlike, whereas in the case of pure line D, twenty-four Paramecia, all measurably different from each other in size, would be found to produce twenty-four progenies practically identical. Organisms that appear to be alike, regardless of their germinal constitution, are said by Johanssen to be identical phenotypically, or to belong to the same phenotype. On the other hand, organisms having identical germinal determiners such as those of the varying members of pure line D, are said to be genotypicaUy alike or to belong to the same genotype. 114 GENETICS Organisms belong to the same phenotype with respect to any character when their somatoplasms are alike. They belong to the same genotype when their germplasms are alike. The word "genotype" was suggested by Johannsen in honor of Darwin and his theory of pangrenesis, although there are certain objections to its use in this connection for the reason that systematists have already appropriated it in a different sense. Natural history and common usage deal prin- cipally with phenotypes, that is, with organisms as they appear. The older theories of heredity were likewise concerned with phenotypes, but we are now coming to see more clearly than before that heredity must always be a case of similarity in origin, that is, in germinal composition, and that similarity in ap- pearance by no means always indicates similarity in origin or true relationship. The assumption that similarity in appearance does indicate relationship has been made the foundation of many conclusions in comparative anatomy and phylogeny, but to the modern student of genetics who places his faith in things as they are, rather than in things as they seem to be, conclusions based upon phenotypical distinctions alone have in them a large source of error which must be taken into account. In a museum of heredity, should such a collection ever be assembled, the specimens would not be ar- ranged phenotypically as they are in an ordinary museum where things that look alike are placed together as if in bonds of relationship, but they THE PURE LINE 115 would be arranged historically from a genetic point of view to show their true origin one from another. 9. THE DISTINCTION BETWEEN A POPULATION AND A PURE LINE A mixture of pure lines has been called a popula- tion. It is not possible to distinguish a pure line from a population by inspection, since both may be pheno- typically alike. Fluctuations about the average occur in both cases with no appreciable difference in character, although such fluctuations, when they occur within a pure line, are simply somatic differ- ences caused in general probably by modifications in nutrition or some other external factor of environ- ment, while fluctuations in a population include not only modifications of this transient nature, but also permanent hereditary differences due to germinal differences in the various pure lines of which the population is composed. Johannsen has made the distinction between pure lines and populations clear by the following figure (Fig. 40), in which five pure lines of beans are com- bined artificially to form a population. The beans which make up the pure lines noted in this figure are represented inclosed within inverted test tubes. The beans in any single tube are all of one size. Tubes vertically superimposed upon each other also contain only beans of one size. Thus it is seen that what may be a rare size of bean in one line, for instance that in the left-hand 116 GENETICS PURE LINE tube of pure line 3, may be identical with the com- monest size in another line, as pure line 2. The five pure lines represented in Figure 40 are combined in a population at the bottom of the figure, making a phenotype that marks the five phenotypes above, which are also five geno- types. In the population, how- ever, the five genotypes are hidden within one phenotype. Hence, while selection within a pure line has no hereditary in- fluence, it is evi- dent that selec- An p.. ,. tion within a 1 IG. 40. Diagrams showing five pure lines and a population formed by their union. The population may beans of each pure line are represented as as- -i -P. sorted into inverted test tubes making a curve Snllt HOVe of fluctuating variability. Test tubes contain- over the type ing beans of the same weight are placed in the . , same vertical row. After Johannsen. Ol tnC progeny POPULATION THE PURE LINE 117 obtained, in the direction of the selection simply by isolating out a pure line of one type. Thus beans chosen from the extreme left-hand test tube in the population cited would belong only to pure line 2, while those taken from the extreme right- hand test tube could belong only to pure line 3. Galton's "law of regression," namely, that minus parents give minus offspring and plus parents plus offspring, with a tendency to reversion from genera- tion to generation, depends simply upon a partial but not complete isolation of pure lines out of a population. From this distinction between pure lines and popu- lations it is clear why breeders in selecting for a particular character out of their stock need to keep on selecting continually in order to maintain a cer- tain standard. As soon as they cease this vigilance, there is a "reversion to type" or, as they say, "the strain runs out," which means that the pure lines become lost in the mixed population which inevi- tably results as soon as selective isolation of the pure line ceases. Such reversion must always be the case in dealing with a population made up of a mixture of pure lines, for only by the isolation of pure lines can the constancy of a character be maintained. When, however, a pure line is once isolated, then all the mem- bers of it, large as well as small, are equally efficient in maintaining the pure line in question, regardless of their phenotypical constitutions. 118 GENETICS 10. PURE LINES AND NATURAL SELECTION From the foregoing statements it appears that by means of selection within a population, such as occurs normally in nature, it is not possible to get anything out that was not already there to begin with. If this is so, the origin of species cannot have come about, as Darwin thought, through natural selection by a gradual accumulation of slight favorable varia- tions. The best that selection can do is to isolate pure lines. Within pure lines it is quite powerless to change the genotypical characters. In other words, natural selection can only maintain and strengthen the frontier posts that are already es- tablished. It cannot break into the wilderness and create new centers. Since the extreme members of a pure line, having the same genotypical constitution, always tend to backslide to mediocrity within the limits of the line in question, the crucial question is : How can the critical step from one genotype to another, a step indispensable in the evolutionary derivation of species, ever occur ? That it has repeatedly oc- curred in the course of time is amply proven by the fact that somehow or other we have gone from Ameba to man. At present the only loophole of escape seems to lie either in the unlikely inheritance of acquired char- acters, or in mutations which make the leap from one character to another, and so eventually from one type to another, without the aid of selection. THE PURE LINE 119 It is interesting to note that Johannsen himself, who has been so prominently concerned in erecting this barrier in the way of the evolutionary derivation of species by natural selection, has recently reported mutations arising within his pure lines of beans. It must be admitted that to the skeptical there is a vicious circle here, for when a variation fails to re- appear in a subsequent generation, it may be ex- plained as the failure of natural selection to act within a pure line, but when a variation does reap- pear it is hailed as a mutation ! In any event the way of experiment lies open, and the evidence of investigators in this critical field will be awaited with keen interest. CHAPTER VII SEGREGATION AND DOMINANCE 1. METHODS OF STUDYING HEREDITY MODERN studies in heredity have been pursued principally in three directions : first, by microscopical examination of the germ-cells ; second, by statistical consideration of data bearing upon heredity; and third, by experimental breeding of animals and plants. The first two of these methods of approach have already been touched upon as well as experimental breeding with reference to "pure lines." In the present chapter attention will be directed to a con- sideration of experimental breeding with reference to hybridization, that is, breeding from unlike par- ents, a process which Jennings characterizes by the expressive phrase, "the melting-pot of cross-breed- ing." 2. THE MELTING-POT OF CROSS-BREEDING Hybridization, or cross-breeding, as formulated by Gal ton (1888), results in one of three kinds of inheritance, namely, blending, alternative, or par- ticulate. 120 SEGREGATION AND DOMINANCE 121 Of these, blending inheritance may be called the typical "melting-pot" in which contributions from the two parents fuse into something intermediate and different from that which was present in either parent. Galton illustrated this process by the inheritance of human stature in which a tall and a short parent produce offspring intermediate in height. A more thorough consideration of this type of inheritance will be presented in Chapter IX. By the method of alternative inheritance the pa- rental contributions do not melt upon union, but retain their individuality, reappearing intact in the offspring. In inheritance of human eye-color, for ex- ample, the offspring usually have eyes colored like those of one of the parents when the parental eye- color is unlike in the two cases, rather than eyes intermediate in color between those of both parents. According to Galton particulate inheritance results when the offspring present a mosaic of the parental characters, that is, when parts of both the maternal and paternal characters reappear in the offspring without losing their identities by blending or without excluding one another. Piebald races of mice arising from parents with solid but different colors have been cited as illustrations of this sort of inheritance, although it will be seen later in connection with the "factor hypothesis" that another interpretation of this phenomenon is not only possible but probable. The distinctions between these three categories of inheritance are diagrammatically represented in Figure 41. 122 GENETICS In blending inheritance the offspring are seen to be unlike either parent, because the parental deter- miners fuse into a new thing. In alternative in- heritance, on the contrary, the offspring may be like either parent, since the characters in question do not lose their individuality upon union, as shown in the diagram. Only one or the other of the two BLENDING ALTERNATIVE PARTICIPATE Characteristics of parents! Qermptasm as shown in the somolopl8m Double g9rm plasm termed from contributions FiQ. 41. Three kinds of inheritance described by Galton. mutually exclusive characters thus becomes effective in determining the nature of each offspring. Finally, in particulate inheritance the double germplasm which determines a new individual may be imagined to undergo a diagonal rather than a vertical cleavage upon maturation, thereby causing unblended fragments of both parental characters to become effective at once, in this manner producing a mosaic offspring. SEGREGATION AND DOMINANCE 123 3. JOHANN GREGOR MENDEL Our understanding of the working of inheritance in hybridization we owe largely to the unpretentious studies of an Austrian monk, Johann Gregor Mendel, who, although a contemporary of Darwin, was prob- ably unknown to him. For eight years Mendel carried on original experiments by breeding peas in the privacy of his cloister garden at Briinn and then sent the results of his work to a former teacher, the celebrated Karl Nageli, of the University of Vienna. At the time Nageli's head was full of other matters, so that he failed to see the significance of his old pupil's efforts. However, in 1866 Mendel's results appeared in the Transactions of the Natural History Society of Briinn, 1 an obscure publication that reached hardly more than a local public. Here Mendel's investigations were buried, so to speak, because the time was not ripe for a general apprecia- tion or evaluation of his work. At that time neither the chromosome theory nor the germplasm theory had been formulated. More- over, much of our present knowledge of cell structure and behavior was not even in existence. Weismann had not yet led out the biological children of Israel through the wilderness upon that notable pilgrimage of fruitful controversy which occupied the last two decades of the nineteenth century, and the attention of the entire thinking world was being monopolized 1 Verhandlungen naturf. Verein in Briinn. Abhandl. IV, 1865 (which appeared in 1866). 124 GENETICS by the newly published epoch-making work of Charles Darwin. Mendel died in 1884, and his work slumbered on until it was independently discovered almost simul- taneously by three botanists whose researches had been leading up to conclusions very much like his own. These three men were de Vries of Holland, von Tschermak of Austria, and Correns of Germany. Their papers were published only a few months apart in 1900 and were closely followed by important papers from Bateson in England and Davenport and Castle in America, with a rapidly increasing number from other biologists the world over. To- day the literature upon this subject has grown to be very large, and the end is by no means yet in sight. Concerning Mendel, Castle has well said: "Mendel had an analytical mind of the first order which en- abled him to plan and carry through successfully the most original and instructive series of studies in heredity ever executed." 4. MENDEL'S EXPERIMENTS ON GARDEN PEAS What Mendel did was to hybridize certain varie- ties of garden peas and keep an exact record of all the progeny, in itself a simple process but one that had never been faithfully carried out by any one. Before examining Mendel's results it may be well to state the difference between normal and artificial self-fertilization. Self-fertilization occurs when from the pollen and ovule of the same flower are derived SEGREGATION AND DOMINANCE 125 the two gametes which uniting produce a zygote that develops into the seed and subsequently into the adult plant of the next generation. In artifi- cially crossing normally self-fertilized flowers it is necessary to carefully remove the stamens from one flower while its pollen is still immature, and later, at the proper time, to transfer to it ripe pollen from another flower. Mendel's cross-breeding experiments on peas showed certain numerical relations which gave rise to what has come to be rather indefinitely known as "Mendel's law." This law may be temporarily formulated as follows : When parents that are unlike with respect to any character are crossed, the progeny of the first gen- eration will apparently be like one of the parents with respect to the character in question. The parent which impresses its character upon the off- spring in this manner is called the dominant. When, however, the hybrid offspring of this first generation are in turn crossed with each other, they will produce a mixed progeny, 25 per cent of which will be like the dominant grandparent, 25 per cent like the other grandparent, and 50 per cent like the parents resem- bling the dominant grandparent. An illustration will serve to make plain the man- ner in which this law works out. Mendel found that when peas of a tall variety were artificially crossed with those of a dwarf variety, all the resulting offspring were tall like the first parent. It made no difference which parent was 126 GENETICS selected as the tall one. The result was the same in either case, showing that the character of tallness is independent of the character for sex. When these tall cross-bred offspring were subse- quently crossed with each other, or allowed to pro- duce offspring by self-fertilization which amounts to the same thing, 787 plants of the tall variety and 277 of the dwarf kind were obtained, making approx- imately the proportion of 3 to 1. On further breeding the dwarf peas thus derived proved to be pure, producing only dwarf peas, while the tall ones were of two kinds, one third of them "pure," breeding true like their tall grandparent, and two thirds of them "hybrid," giving in turn the proportion of three tall to one dwarf like their parents. These crosses may be expressed as follows : Tall, T, X dwarf, t, = tall, T(t). That is, tallness crossed with dwarfness equals tallness with the dwarf character present but latent. Mendel termed the character, which became ap- parent in such a hybrid, in this case tallness, the dominant, and the latent character which receded from view, in this instance dwarfness, the recessive. When now the hybrids, T(t\ were crossed to- gether, the result algebraically expressed was as follows : T + 1 (all possible egg characters) T + t (all possible sperm characters) TT+ Tt Tt +tt TT+tT(t)+tt SEGREGATION AND DOMINANCE 127 MALE GAMETES T i T t That is, one out of four possible cases was dwarf, t, in character and the other three were apparently tall, although only one out of the three was pure tall, T, while the remaining two were tall with the dwarf character latent, T (). The same thing may be expressed more graphically by the checkerboard plan, which Punnett suggested (Fig. 42). Each square of the checkerboard rep- resents a zygote which, having received a gamete from each of the two par- ents, may develop into a possible offspring. The character of the gametes of the parents is shown outside of these squares, while the arrows repre- sent the parental source from which the offspring have received their heredi- tary composition. The essential feature of Mendel's law is briefly this: hereditary characters are usually independent units which segregate out upon crossing, regardless of temporary dominance. Mendel carried on further experiments with garden peas, using other characters. He obtained practically the same result as in the instance already given, for the actual progeny in the second generation of the cross-bred offspring figured up, as seen in the table * >TT 4. Ti >tT i i FIG. 42. Diagram to illustrate theoretically the formation of the four possible zygotes in the second filial generation of a monohybrid. 128 GENETICS below, very nearly to the expected theoretical ratio of 3 to 1. CHARACTER NUMBER OF DOMINANTS NUMBER OF RECESSIVES RATIO Form of seed .... 5474 smooth 1850 wrinkled 2.96 to 1 Color of seed coat . . Color of flowers . . . 6022 yellow 705 colored 2001 green 224 white 3.01 to 1 3.15 to 1 Form of pods .... Color of unripe pods . . Position of flowers 882 inflated 428 green 651 axial 299 constricted 152 yellow 207 terminal 2.95 to 1 2.82 to 1 3.14 to 1 Length of vine . . . Total 787 tall 277 dwarf 2.84 to 1 2.98 to 1 Darbishire repeated the yellow-green cross with garden peas, obtaining in the second generation the large total of 139,837 individuals of which 105,045 were yellow and 34,792 green, which is very close to 3 tol. 5. SOME FURTHER INSTANCES OF LAW" 'MENDEL'S Since the rediscovery of Mendel's law the ratio of 3 to 1 in the second generation has been found by a number of different investigators to be constant in a large array of characters observed both in animals and plants of diverse kinds when these are cross-bred with reference to the characters in question. Botanists have the advantage perhaps in this matter, as they deal with forms which usually produce a large number of offspring from a single cross, a very desirable thing in estimating ratios. On the SEGREGATION AND DOMINANCE 129 other hand, they are handicapped by being unable usually to obtain more than one generation in a year, while zoologists may secure from many animals like rabbits and mice several generations in a year, al- though ordinarily the number of progeny is much ORGANISM AUTHOR DOMINANT RECESSIVE 1 Q Nettles Correns Serrated leaves Smooth-margined leaves '03 Sunflower Shull Branched habit Unbranched habit '08 Cotton Balls Colored lint White lint '07 Snapdragon Baur Red flowers Non-red flowers '10 Wheat Biffen Susceptibility to Immunity to rust '05 rust Tomato Price and Two-celled fruit Many-celled fruit '08 Drinkard Maize de Vries Round, starchy Wrinkled, sugary kernel '00 kernel Silkworm Toyama Yellow cocoon White cocoon '06 Cattle Spillman Hornlessness Horns '06 Pomace fly Morgan Red eyes White eyes '10 Horses Bateson Trotting habit Pacing habit '07 Land snail Lang Unhanded shell Banded shell '09 Mice Darbishire Normal habit Waltzing habit '02 Guinea-pig Castle Short hair Angora hair '03 Canaries Bateson and Crest Plain head '02 Saunders Poultry Davenport Rumplessness Long tail '06 Man Farrabee Brachydactyly Normal joints '05 Barley von Tschermak Beardlessness Beardedness 01 Salamander (Amblystoma) Haecker Dark color Light color '08 smaller and the ratios obtained have a larger chance of error than is the case with the more numerous plant offspring. Semi-microscopic animals, as, for example, the pomace fly, Drosophila, which produces a large progeny every two weeks or so, may combine the general advantages mentioned for the two groups of organisms 130 GENETICS indicated above, but they have the disadvantage of being so small that the detection of their distinctive phenotypic characters is attended with considerable technical difficulty. What the modern experimenter in genetics desires is an organism, first, that possesses conspicuous distinc- tive somatic characters, and, second, which will come to sexual maturity early and breed either in captivity or under cultivation both numerously and frequently. The preceding table, compiled chiefly from Bateson 1 and Baur, 2 might easily be much extended. It shows from what diverse sources confirmatory evidence of the truth of Mendel's law has been derived within the past few years. 6. THE PRINCIPLE OF SEGREGATION The essential thing which Mendel demonstrated was the fact that, in certain cases at least, the deter- miners for heredity derived from diverse parental sources may unite in a common stream of germplasm from which, in subsequent generations, they may segregate out apparently unmodified by having been intimately associated with each other. This "law of segregation" depends upon the conception that the individual is made up of a bundle of unit characters. It may be illustrated by the separate flowers picked from a garden which, after being made into a nose- gay, may be taken apart and rearranged without in l " Mendel's Principles of Heredity," 1909. '"Einfuhrung in die experimentelle Vererbungslehre," 1911. SEGREGATION AND DOMINANCE 131 any way disturbing the identity of the separate blossoms. The general formula of segregation that covers all cases of organisms cross-bred with respect to a single character, that is, monohybrids, is given in Figure 43. (DoniMwit) K (Rtcessi.tl OD D(R) DD 2D(R) RR JDD *D(R) RR DD DD DD 2 DCR) RR RR RR FIG. 43. General Mendelian formula for a monohybrid. 7. HOMOZYGOTES AND HETEROZYGOTES A character which is present in the offspring in double quantity because it was present in both parents is said by Bateson to be homozygous, while an or- ganism which is homozygous with respect to any character is called a homozygote so far as that particu- lar character is concerned. In contrast to the homozygous condition, an organ- ism is said to be heterozygous when it derives the determiner of a character from one parent only. Such an organism is described as a heterozygote with respect to the character in question. A homozygous and a heterozygous dominant may appear alike, 132 GENETICS although not necessarily so, that is, they may have the same phenotypical constitution, but their geno- typical composition is always different. 8. THE IDENTIFICATION OF A HETEROZYGOTE "Homozygote" and " heterozygote " are terms then descriptive solely of the genotypical constitution of organisms, and, as has been said, it is not always possible to distinguish one from the other by inspec- tion, although it may frequently be done, as will be pointed out later. The only sure way to identify a heterozygote is by breeding to a recessive and observing the kind of offspring produced. Peas of the formulae TT and T(), for example, both look alike, since a single determiner for the tall character, T, is sufficient to produce complete tallness. When, however, these two kinds of tall peas are each bred to a recessive dwarf pea, of the formula tt, the progeny will differ distinctly in the two cases as follows : Case I. T + T X t + t = 100 per cent T(f). Case II. T + t X t + t = 50 per cent T(t) + 50 per cent tt. That is, if the dominant to be tested is homozygous (Case I), the entire progeny will exhibit the dominant character, but if the dominant to be tested is heterozy- gous (Case II), then only one half of the progeny will show the character in question. 9. THE PRESENCE AND ABSENCE HYPOTHESIS Mendel's conception that every dominant character is paired with a recessive alternative is now being SEGREGATION AND DOMINANCE 133 largely replaced by the presence and absence hypothesis which was first suggested by Correns but later logi- cally worked out by others, particularly by Hurst, Bateson, and Shull. According to this latter inter- pretation, a determiner for any character either is, or is not, present. When it is present in two parents, then the offspring receive a double, or duplex, "dose," to use Bateson's word, of the determiner. When it is present in one parent only, then the offspring have a single, or simplex, dose of the character. When it is present in neither parent, it follows that it will not appear in the offspring. In this case the offspring are said to be nulliplex with respect to the char- acter in question. Take the case of tall and dwarf peas, the determiner for tallness when present pro- duces tall peas, even if it comes from one parent only, but if this determiner for tallness is absent from both parents, the offspring are nulliplex, that is, the absence of tallness results and only dwarf peas are produced. The difference between the presence and absence theory and the dominant and recessive theory is that in the former case the "recessive" character has no existence at all, while in the latter instance it is present, but in a latent condition. 10. DIHYBRIDS So far reference has been made exclusively to mono- hybrids, any two of which are supposed to be similar except with respect to a single unit character. Mono- hybrids are comparatively simple, but when two 134 GENETICS organisms are crossed which differ from each other with respect to two different unit characters, the situa- tion becomes more complicated. Mendel solved the problem of dihybrids by crossing wrinkled-green peas with smooth-yellow peas. He found that smoothness S is dominant over wrinkled- ness W and that yellow color Y is dominant over green G, or, as it would be stated according to the presence and absence theory, smoothness is a positive character which fills out the seed-coat to plumpness while its absence leaves a wrinkled coat, and yellow- ness is a positive character due to a fading of the green which causes the yellow to be apparent. In the absence of this green fading factor or determiner the green, of course, appears. If smooth-yellow SY and wrinkled-green WG are crossed, all the offspring are smooth-yellow, but they carry concealed the recessive determiners for wrinkledness and greenness according to the formula S(W)Y(G). When the determiners of these cross- breds segregate out during the maturation of the germ-cells, they may recombine so as to form four possible double gametes, namely, smooth-yellow SY and wrinkled-green WG, which are exactly like the grandparental determiners from which they arose, and in addition, two entirely new combinations, smooth-green SG and wrinkled-yellow WY. Since the male and the female cross-breds are each furnished with these four possible gametic combina- tions, the possible number of zygotes formed by their union will be sixteen (4X4 = 16). That is, the SEGREGATION AND DOMINANCE 135 monohybrid proportion of 3 to 1 in dihybrid com- binations is squared, (3 + I) 2 = 16. It of course does not follow that the offspring in dihybrid crosses will always be sixteen in number, or that they will always conform strictly to the theoreti- cal expectation of (3 + I) 2 . The offspring obtained undoubtedly obey the laws of chance, but the greater the number of offspring, the nearer they come to fall- ing into the expected grouping. The sixteen possible zygotes resulting from a dihybrid cross will give rise to sixteen possible kinds of individuals which in turn, as will be demonstrated directly, present four kinds of phenotypic and nine kinds of genotypic constitutions. A dihybrid mating, using the same symbols em- ployed in the case just described, would be expressed algebraically as follows : SG+ WY+ SY+ WG =- all the possible egg gametes SG+ WY+ SY+ WG = a\l the possible sperm gametes SGSG+ SGWY+ SGSY+ SGWG SGWY +WYWY+ WYSY+ WYWG SGSY + WYSY +SYSY+ SYWO SGWG + WYWG + SYWG+WG^G SGSG+2 SGWY+2 SGSY+2 SGWO+WYWY+2 WYSY+ 2 WYWG+SYSY+ 2SY WG+ WO* G The second and the ninth items in this result are alike ; by combining them the revised result reads : 8GSG+4SGWY+2SGSY+2SGWG+WYWY+2 WYSY+2 WYWG+SYSY+WGWG There are then these nine different combinations of germinal characters or nine different genotypes in any dihybrid cross. By placing the recessive char- acters in parentheses, whenever the corresponding dominant is present to indicate that the dominant 136 GENETICS causes the former to recede from view, these nine genotypes may be combined into four phenotypes as follows : Phenotypes . . 9SF 3SG 3WY IWG Genotypes . . 4S(G)(PF)F 2S(G)SF tSY(W)Y SYSY SGSG 2SG(W)G WYWY 2 WYW(G) WGWQ From this analysis it may be said that the Mendelian ratio for a typical dihybrid is phenotypically 9:3:3:1, while that for a monohybrid, as we have already seen, (?- SG WY SY WG 9 I 4 * * * SG WY SY WG SG- SG SG SG SG (D SG WY SY WG WY- WY WY WY WY (D (D (D SG WY SY WG SY- SY SY SY SY WG- SG WG WY WG SY WG WG WG (3> @ FIG. 44. Diagram to illustrate the possible combinations arising in the second filial generation (F 2 ) following a cross between yellow-smooth YS and green-wrinkled GW peas. is 3:1. This expected ratio corresponds essentially with the actual results Mendel obtained in crossing smooth-yellow and wrinkled-green peas. SEGREGATION AND DOMINANCE 137 Figure 44 presents a graphic representation of the different combinations resulting from a dihybrid cross following the checkerboard plan used in Figure 42 to illustrate monohybrids. The nine genotypes and four phenotypes which result from a dihybrid cross are shown in the following squares. Number in Each Class GENOTYPE Number of Squares in Fig. 44 PHENOTYPE Number in Each Class 1 SYSY 11 SY 9 2 (W)YSY 7-10 2 S(G)SY 3-9 4 S(G)(W)Y 2-5-12- 15 1 SGSG 1 SG 3 2 SG(W)G 13-4 1 WYWY 6 WY 3 2 WYW(G) 8-14 1 WGWG 16 WG 1 16 16 Another illustration of dihybridism is shown in Figures 45 and 46 which is based upon data fur- nished by the Davenports. 1 In the matings given here, dark or pigmented hair, represented by the solid black circles, is dominant over light-colored, that is, unpigmented or slightly pigmented hair, symbolized by the open circles, while curly hair is dominant 1 "Heredity of Eye-color in Man," Science, N. S. 26, p. 589, 1907; " Heredity of Hair Form in Man," Amer. Nat. 42, p. 341, 1908. Daven- irt, C. B. and G. C. 138 GENETICS over straight, represented by crooked and straight lines respectively in the diagram. In other words, the presence of pigment is dominant over the ab- sence of pigment, while the factor that causes curli- ness is dominant over the absence of this factor, with respect to human hair. FIQ. 45. The heredity of human hair according to data by C. B. and G. C. Davenport. The arcs represent the somatoplasms of four indi- viduals. Within the arcs are the gametes formed by these individuals. The dominant character is placed on the outside of the arc where it will be visible. SEGREGATION AND DOMINANCE 139 When a homozygous individual with dark curly hair crosses with a homozygous individual with light straight hair, all the offspring have dark curly hair. The dark curly-haired individuals of this second generation, however, are heterozygous with respect to each of these two hair characters. When any two individuals having this particular genotypic compo- sition mate, therefore, they may produce any one of four possible phenotypes dark curly, dark straight, light curly or light straight haired individ- uals. These four phe- notypes in turn will present nine different genotypic combina- tions out of sixteen pos- sible cases, as shown in Figure 46. Figure 45 further- more serves to make clear, first, the distinc- tion between somato- plasm and germplasm; second, the maturation of germ-cells ; third, the segregation of gametes ; and fourth, the formation of zygotes in sexual reproduction. The cells of the somatoplasm are represented as N *h GENOTYPE PHENOTYPC Nuk.r f Dark curly 9 2 (QS) ; i Dark straight 3 / bght curly 5 f / $3) Light straight 1 IQ /6 FIQ. 46. Diagrams showing the pos- sible genotypic and phenotypic com- binations resulting when two hetero- zygous individuals, with dark curly hair, mate. Symbols are the same as in Figure 45. 140 GENETICS making up the arcs within which are inclosed the germ-cells after their reduction through maturation, which results in giving to each germ-cell half the number of determiners that are present in the soma- tic cells. It will be remembered that when two gametes, or mature germ-cells, unite, they form a zygote having the proper number of determiners normal to the species in question instead of double that number. Symbols for dominant characters in the diagram are placed on the outside of the somatic arcs, because these are the characters that are visible or pheno- typic, while the non-apparent recessives are placed on the inside out of sight. 11. THE CASE OF THE TRIHYBRID Mendel went even further and computed the possibilities which would result when two parents were crossed differing from each other with respect to three unit characters. He found that the results actually obtained by breeding closely approximated the theoretical expectation. This expectation in the case of a trihybrid cross is that the cross-breds resulting will all exhibit the three dominant characters, while their genotypic constitution will include six factors, namely, these three dominant characters plus their corresponding recessives or "absences." Cross-breds of the first generation will, therefore, have eight possible kinds of triple gametes and when interbred may form a possible range of sixty-four SEGREGATION AND DOMINANCE 141 (8 X 8) different zygotes, which corresponds to a monohybrid raised to the third power (3 + I) 3 . These sixty-four zygotes group together in eight cf- RSP- RsP- RSp- Rsp - rSP- reP- r Sp rsp RSP R*P RSp R.p rSP rP r SP r*p I 1 1 I I I I I RSP RSP R 8 P RSP RS P RSP Rsp RSP rSP RSP rsP RSP rSp RSP rsp RSP RSP RsP RsP RsP RSp RsP Rsp RsP rSP RsP rsP RsP rSp RsP rsp RsP RSP RS P RsP RS P RSp RSp Rsp RS P rSP RSp rsP RSp rSp RSp r P RSp RSP Rp RsP Rsp RS P Rsp Rsp Rep rSP Rsp reP Rsp rSp Rsp rep Rap RSP rSP RsP rSP RSp rSP Rsp rSP rSP rSP rsP rSP rSp rSP rap rSP RSP rsP RsP rsP RS P reP Rep re.P rSP rsP rsP rsP rSp rsP rep rsP RSP rSp RsP rSp RSp rSp Rsp rSp rSP rSp reP rSp rSp rSp rsp rSp RSP rap ReP rsp RSp rsp Rep rsp rSP rep reP rsp rSp rsp rsp rap FIG. 47. Diagram showing the possible combinations in a guinea-pig trihybrid of the Fa generation. R, resetted coat ; r, non-rosetted coat (absence of K) ; S, short hair ; s, angora hair (absence of S) ; P, pig- mented ; p, albino (absence of pigment). The eight possible triple gametes of each parent are placed in the upper and left hand margins. Each of the sixty-four squares represents a possible zygote or ferti- lized egg, having received a triple gamete from each parent. different phenotypes and twenty-seven different genotypes. The trihybrid cross with its resulting combinations is well illustrated by Castle's work on guinea-pigs which confirms the Mendelian hypothesis on an 142 GENETICS Number in each class GENOTYPE PHENOTTPE Number in each class 1 SS PP RR SPR Short, pigmented, resetted 27 2 SS Pp RR 2 Ss PP RR 4 Ss Pp RR 2 SS PP Rr 4 SS Pp Rr 4 Ss PP Rr 8 Ss Pp Rr 1 SS pp RR SpR Short, albino, resetted 9 2 Ss pp RR 2 SS pp Rr 4 Ss pp Rr 1 ss PP RR sPR Angora, pigmented, resetted 9 2 ss Pp RR 2 ss PP Rr 4 ss Pp Rr 1 SS PP rr SPr Short, pigmented, non-rosetted 9 2 SS Pp rr 2 Ss PP rr 4 Ss Pp rr 1 ss pp RR spR Angora, albino, resetted 3 2 ss pp Rr 1 SS pp rr Spr Short, albino, non-rosetted 3 2 Ss pp rr 1 ss PP rr sPr Angora, pigmented, non-rosetted 3 2 ss Pp rr 1 ss pp rr spr Angora, albino, non-rosetted 1 64 64 SEGREGATION AND DOMINANCE 143 extensive scale. In Figure 47 dominant characters are represented by capital letters, while recessives or absences are indicated by corresponding small letters. When a smooth, or non-rosetted (r), short-haired (S), pigmented (P) guinea-pig is crossed with a resetted (R), long-haired (s), albino (p) guinea-pig, all the offspring appear to be of one phenotypic constitution, namely, resetted, short-haired, and pigmented (RSP). Their genotypic constitution is represented by the formula RrSsPp. These six factors may form eight possible triple gametes, as follows: RSP,RsP,RSp,Rsp,rSP,rsP,rsp. When two germ-cells each made up of these eight triple gametes unite in sexual reproduction, they will give rise to sixty-four (8 X 8) possible zygotes as dis- played in Figure 47. An analysis of Figure 47 shows among the off- spring eight different phenotypes in the ratio of 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 and 27 different genotypes in the proportions indicated on the opposite page. The order of the three pairs of symbols is changed from that in Figure 47 to emphasize the fact that with independent unit characters the order is immaterial. 12. CONCLUSION Although the ratios for more than a trihybrid were computed by Mendel, the experimental test has never been carried out, since it involves such large and complicated proportions. In the case of four differing unit characters in the parental generation, the offspring of the quadruple 144 GENETICS hybrids derived from such an ancestry would in- clude 256 or (3 + 1) 4 possibilities instead of 64 or (3 + 1) 3 , as in the case of trihybrids. When ten differing characters are combined in the parental generation, there would result over a million possible kinds of offspring among the hybrids of the second generation, (3 + 1) 10 = 1,048,576. From the foregoing it is apparent that in practical breeding the only hope lies in dealing with not more than one or two characters at a time. Since unit characters usually behave independently of each other, one may breed for a single character until it is segregated out in a homozygous, that is pure, condition, and then in the same way obtain a second character, a third, and so on. Thus in a few generations of properly directed crosses there can be obtained combinations of char- acters united in one strain that formerly were never obtained at all or were only hit upon by the merest chance at long intervals. Herein lies the scientific control of heredity which the trinity of Mendelian principles : namely, independent unit characters, seg- regation, and dominance, has placed in human hands. 13. SUMMARY Three principles are concerned in Mendel's law: independent unit characters, dominance, and seg- regation. a. Independent Unit Characters. An organism, although acting together as a physiological and mor- phological whole, may be regarded from the point SEGREGATION AND DOMINANCE 145 of view of heredity as consisting of a large number of independent heritable unit characters. b. Dominance. In the germplasm there are cer- tain determiners of unit characters which dominate others during the development of the somatoplasm. In other words, they determine the apparent charac- ter of the organism by causing that character to become visible. The alternative recessive characters, although they may be present in the germplasm, are unable to be- come manifest in the somatoplasm so long as the dominant characters are present. When, however, a dominant character is absent, its recessive alterna- tive becomes manifest. c. Segregation. Unit characters, although they may be intimately associated together in the indi- vidual, during the complicated process of maturation that always precedes the formation of a new indi- vidual, separate or segregate out as if independent of each other and thus are enabled to unite into new combinations. CHAPTER VIII 1. THE DISTINCTION BETWEEN REVERSION AND ATAVISM THERE are two ways in which types of animals or plants that are different from the present ones may be conceived to arise, namely, by the reappearance of old types and by the formation of new ones. In the reappearance of old types a distinction may be drawn between reversion and what has been termed atavism. Atavism, or "grandparentism," may be defined as skipping a generation with the result that a particu- lar character in the offspring is unlike the corre- sponding character in either parent, but instead, resembles the character in one of the grandparents. In reversion, on the contrary, a character reappears which has not been manifest perhaps for many gen- erations, although it was actually present in some remote ancestor. J. Arthur Thomson's definition of reversion is : "All cases where through inheritance there reappears in an individual some character which was not expressed in his immediate lineage, but which had occurred in a remoter, but not hypo- thetical, ancestor." 146 OLD TYPES AND NEW 147 This distinction between atavism and reversion becomes clearer by illustration. If heterozygous brown-eyed individuals mate, there is one possibility in four that their offspring GRANDMOTHER GRANDFATHER HOMOZYGOTE Duplex, I \ HETEROZYGOTE HOMOZYGOTE HETEROZYGOTE Simplex Duplex v _ fj .. Simplex FIQ. 48. Three generations of a Mendelian monohybrid. The outlines represent the somatoplasms with the phenotypic character on the out- side. The black symbols inclosed within the somatoplasm stand for the germplasm in the form of gametes. The short dotted arrows indi- cate the relation between germplasm and somatoplasm. The long dotted arrows indicate possible recombinations of germplasms. will have blue eyes unlike their own, but like the two blue-eyed grandparents. Such a blue-eyed child would be an instance of atavism. The explanation 148 GENETICS of this apparently inconsistent hereditary behavior is perfectly simple in the light of the Mendelian ratios, as shown diagrammatically in Figure 48, in which the circles represent the blue-eyed and the squares the brown-eyed character. This figure also illustrates what typically occurs in the formation of Mendelian monohybrids of the first and second filial generations. The squares are symbols for the dominant characters, while the circles are symbols for the recessive characters. When the two are superimposed, the circle recedes from view, The large outside figures indicate the somatoplasm, therefore the phenotype. The small inclosed figures indicate the germplasm, therefore the genotype. The short dotted arrows indicate what it is that deter- mines the somatoplasm in each case, while the long dotted arrows show what possible recombinations of germplasms can be made. Child No. 4 is an " ex- tracted recessive" derived from dominant parents, but with one recessive grandparent on each side. It is a case of "atavism," or taking after the grand- parent. Notice that atavism can occur only by alternative inheritance. To quote Davenport: "In the majority of cases atavism is a simple reappearance in one fourth of the offspring of the absence of a character due to the simplex nature of the character in both parents." An illustration of reversion would be the reappear- ance of the ancestral jungle-fowl pattern in domestic poultry or of the slaty blue color of the ancestral rock-pigeon among buff and white domestic pigeons, OLD TYPES AND NEW 149 for the ancestral character or characters in this type of hereditary behavior, as said before, reappear only after a lapse of many generations. 2. FALSE REVERSION "Around the term * reversion,' " Bateson observes, "a singular set of false ideas have gathered them- selves. ' ' In proof of this statement there may be cited at least five categories of apparent reversion which properly ought not to be classed as true reversion. a. Arrested Development Feeble-mindedness is not reversion to ancestral forms of less intelligence, but an instance of arrested development when, for some reason, the individual fails to accomplish his normal cycle of development. Likewise harelip in man is not a case of reversion to rabbit-like ancestors in which harelip is the nor- mal condition, but it is ordinarily due to an arrest or failure of certain embryonic steps that are essential to the development of the usual form of human lip. b. Vestigial Structures These are the vanishing remains of characters that were formerly of significance. They do not represent something latent that is now reappearing, for they have never yet disappeared phylogenetically, and con- sequently they cannot be regarded as true reversions. The muscles under the scalp which enable those persons possessing them to wiggle the ears; the palatine ridges in the roof of the mouth of many 150 GENETICS babies and some adults which resemble the ridges in the roof of a cat's mouth ; the vermiform appen- dix, a necessary part of the digestive apparatus of many animals but fraught so often with evil conse- quences to man ; these and scores of similar charac- ters, which, taken together, make man in the eyes of the comparative anatomist a veritable old curi- osity shop of ancestral relics, are the last traces of characters which formerly had a significance in some of man's forbears. Having lost their usefulness, these structures still hang on to the anatomical household as pensioners. They have not been re- called from the past, but have always been with us, although of diminishing importance. In no sense, therefore, can they be called reversions. c. Acquired Characters resembling Ancestral Ones Sometimes the drunken descendant of a drunken great-grandparent has acquired this characteristic through his own initiative quite aside from any an- cestral contribution to his germplasm. This is not reversion. It is a reacquisition which resembles the ancestral condition. Again, tame animals that run wild acquire habits resembling those of their wild ancestors, but this is not necessarily reversion. It is the natural response of feral animals to the conditions of wild life. d. Convergent Variation The European hedgehog, Erinaceus, an insecti- vore, the American porcupine, Erithizon, a rodent, and the Australian spiny anteater, Echidna, a OLD TYPES AND NEW 151 monotreme, are all mammals which have developed in a similar manner the very peculiar device of der- mal spines. There is no reason, however, for regard- ing this character as due to descent from a common spiny ancestor. It is not reversion to an ancestral type, but rather a case of convergent variation. Similarity does not always indicate genetic continuity. In the case of birds albinism, melanism and fla- vism are modifications of ordinary pigmentation which appear irregularly among many different species as pathological " sports," but no one of these conditions can be regarded as reversions to ancestral white, black, or yellow types. e. Regression Galton's "law of regression" refers to the wide- spread phenomenon already explained of a constant swinging back to mediocrity which the breeder must oppose with continual selection in order to maintain the standard of any particular strain. We have seen that within a "pure line," regression is complete and that in populations made up of a mixture of pure lines it is a factor always to be reckoned with. Regression, however, has to do with fluctuating varia- tions and does not bring about a permanent change of type. It should, therefore, not be confused with reversion. 3. EXPLANATION OF REVERSION Darwin, who did not always differentiate between reversion and atavism, suggested that reversion was 152 GENETICS due sometimes to the action of a more natural en- vironment, as in the case of animals set free after having been in captivity, and sometimes to hybridi- zation, since there seems to be a general tendency of hybridized organisms to "revert" to ancestral types. It is now known that reversion, like atavism, is simply a case of latent characters becoming apparent according to the Mendelian principle of segregation. To quote Davenport : "There is nothing more mys- terious about reversion* from the modern standpoint, than about forming a word from the proper com- bination of letters." 4. SOME METHODS OF IMPROVING OLD AND ESTAB- LISHING NEW TYPES a. The Method of Hallet This method, which was formulated by the English wheat-grower Hallet in 1869, has been in common use for a long time. It consists in placing the organ- isms to be bred in the very best possible environment and then choosing those individuals which make the best showing as the stock from which to breed further, a procedure based upon the deep-seated belief that acquired characters are inherited. For example, in a field of wheat, plants near the edge of the field which, from lack of crowding or by reason of proximity to an extra local supply of fer- tilizer or any other favorable environmental factor, make a more vigorous growth than their neighbors, OLD TYPES AND NEW 153 are selected in the hope that the gains made by them will be maintained in their offspring. We have seen that it is very questionable whether acquired characters which are due to environmental conditions play any role whatever in heredity. The phenotypic character does not always indicate what the germplasm will subsequently do, and when the true genotypic constitution of the germplasm is still further masked by the temporary fluctuations caused by a modified environment, it is increasingly difficult to select wisely from the display of variants those which will produce the best ancestors for the future stock. That this common procedure of selecting the best- appearing animal in the flock and the biggest ear of corn in the bin, has met with a large degree of success in the past is due entirely to the fact that in many instances the phenotypic character is an actual ex- pression of the genotypic constitution. This is not always the case, however, and we cannot now fail to see that the method is blind and full of error. Its successes are due to the indirect results of chance rather than to a direct control of the factors of hered- ity. The great proportion of failures resulting from this procedure now find a reasonable explanation from the standpoint of Mendelism. 6. The Method of Rimpau Contrasted with the Hallet method of augmenting acquired characters and then selecting the best display of them, is the method of Rimpau, who experimented 154 GENETICS for two decades with various grains and, finally, among other results, produced the famous Schland- stedt barley. Rimpau's method is to sow grain under ordinary conditions with a minimum rather than a maximum amount of fertilizer and then to select individuals, neither from the rich spots nor from the edges of the field where there is little crowding, but from situa- tions where the environmental conditions are ordi- nary or even unfavorable. Individuals making a good showing under such usual, or even adverse, conditions are worthy by nature rather than by nur- ture and are consequently most desirable as progeni- tors of future stock. By this method the attempt is not to keep the progeny of single individuals sepa- rate, but to mass together the best as they appear under ordinary normal environment. This again is an indirect method of procedure, although the character of the germplasm is more nearly hit upon in this way than by Hallet's method, since the mask of temporary accessory modifications is stripped so far as possible from the somatoplasm, and the phenotype made to approximate the geno- typical constitution. c. The Method of de Vries The method of de Vries has already been in part described in Chapter IV. It depends upon the pres- ervation and exploitation of the mutations occurring in nature. It recognizes clearly the fact that change of type is dependent upon a germplasmal variation OLD TYPES AND NEW 155 which is largely, if not entirely, independent of environ- mental factors. Accordingly, the work of the successful breeder consists in simply taking what nature spontaneously furnishes to him rather than in attempting to force nature into producing something new. These muta- tions, when isolated, may become the progenitors of desirable new lines. d. The Method of Vilmorin This is an isolation method which has been success- fully applied to the sugar-beet industry. The seeds from each plant to be tested are sown in separate beds from which upon maturity samples are taken and tested for sugar content. The plants from the bed furnishing the sample which contains the highest per- centage of sugar are then used as the seed producers for the next generation. In this way by continual selection an improved strain may be maintained. e. The Method of Johannsen The method of isolating pure lines or homozygotes out of a mixed population has been considered in Chapter VI. As in the method of de Vries of isolating mutations, so, too, in the pure line method it is recog- nized that the germplasm is the source of initiatory changes and that the technique of establishing new types consists in sorting out homozygous strains of this germplasm. The method of Johannsen is quite different from those of Hallet and Rimpau in that the ideal organi- 156 GENETICS zation is not sought for among phenotypes, but among genotypes. It is not the somatoplasm, but the germplasm that is selected. /. The Method of Burbank This is a method of greatly increasing the number of variants by promiscuous hybridization and then of eliminating all except those of a desired phenotypic combination. Indirectly it depends upon the princi- ple of the segregation of unit characters which makes possible rearrangements of these characters according to the laws of chance. The characters themselves remain unchanged, since nothing new is produced by hybridization except new arrangements of existing characters. The spectacular success of Luther Burbank in "creating" new plant forms is due largely to his very extensive hybridizations, his skill in detecting among the varying progeny the winning phenotype and his ruthless elimination of the great majority of varia- tions that do not quite fill his requirement. The successful combinations must be propagated in most instances asexually by grafting, cuttings, bulbs, etc., rather than sexually through the medium of seeds, because new genotypes which will breed true are not necessarily isolated by this procedure. The consequence is that Burbank's method cannot be utilized in animal breeding to any great extent where the maintenance of a desirable strain by asexual prop- agation is out of the question. It will be seen that this method, like the first two, is OLD TYPES AND NEW 157 fortuitous and to a certain extent unscientific in that no one can repeat the exact conditions of the experi- ment and arrive at the same results. It depends upon the chance mixing up of a large number of possibilities and then in not being distracted or blinded by the good while selecting the best. In the hands of a skilful plant breeder with unlimited resources at his com- mand it may result in much practical achievement, but it does not particularly illuminate the path of other breeders who wish to repeat the experiment. It is after all a selection of phenotypes and, therefore, forever open to error, since phenotypes do not always indicate what the behavior of their constituent geno- types will be in heredity. g. The Method of Mendel The method of Mendel, like the foregoing, depends upon hybridization with the difference that the desired combination is sought directly by definite predetermined crosses, according to the expectations of the Mendelian ratios, rather than through the random result of fortuitous combinations. This method has been rendered possible by the determina- tion of Mendel's laws of dominance, and of the inde- pendence and segregation of unit characters which give to the experimental breeder definite expectations and a method of procedure. If, upon hybridization, the desired character be- haves like a recessive, then all that is necessary to establish a pure stock exhibiting the character in question, is to breed two recessives together, because 158 GENETICS recessives are always homozygous and, regardless of their ancestry, breed true. On the other hand, if the desired character proves to be a dominant, then it is necessary to determine whether it is present in a duplex or a simplex condi- tion; in other words, whether it is homozygous or heterozygous, for only homozygous organisms breed true. Establishing a strain consists, consequently, in making an organism homozygous. The test to determine whether a dominant character is homozygous or heterozygous, that is, whether it will breed true or not, can be made by a single cross according to the procedure outlined in para- graph 8 of Chapter VII. If, upon crossing the individual to be tested with a recessive, it produces an entirely dominant progeny, then its germplasm is duplex for this character, and it will always reproduce the character in either duplex or simplex condition according to what it may be crossed with. When crossed, for instance, with another duplex dominant like itself, a pure homozygous strain of the character in question will be perpetuated. If, on the contrary, the dominant character to be tested proves to be simplex or heterozygous, as de- termined by the fact that, when crossed with a re- cessive, 50 per cent of the progeny are recessive, then it requires more than a single generation to establish a homozygous dominant strain. In random inbreeding of diverse strains if the re- cessives are constantly eliminated as they appear, a population is gradually obtained which is composed OLD TYPES AND NEW 159 of an increasing number of dominants so that after only a few generations the chances are much reduced that recessives will appear, which means the practical purity of the strain. 5. THE FACTOR HYPOTHESIS It has been ascertained within the last decade that some characters require more than a single de- terminer to bring them to expression. The idea of compound determiners for a single character may be termed the factor hypothesis of heredity. The con- verse is also true, that certain single determiners may control more than one character. For instance, the determiner for gray hair in rats also produces a lighter color on the belly. Mendel, whose experiments led him to believe that each character depends upon only a single deter- miner for the reason that he worked on characters severally belonging to different parts of the plant, was apparently unaware of the existence, hi certain cases at least, of compound determiners. These compound factors may be arranged in va- rious categories. For example, there may be, (1) A complementary factor which is added to a dissimilar factor in order that a particular character may appear ; (2) A supplementary factor which is added to a dissimilar factor with the result that a character is modified in some way ; (3) A cumulative factor which, when added to 160 GENETICS another similar factor, affects the degree of expression that a character is given ; (4) An inhibitory factor., which prevents the action of some other factor, and so on. It will be profitable to consider the factor hypothe- sis in some detail, since it helps to explain both rever- sion and the formation of new types. a. Bateson's Sweet Peas In the course of numerous breeding experiments Bateson obtained two strains of white sweet peas, Laihyrus, which, when normally self-fertilized, each bred true to the white color. When these two strains were artificially crossed, however, the progeny all had purple flowers like the wild ancestral Sicilian type of all cultivated varieties of sweet peas. Here was apparently a typical instance of reversion, but according to the factor hypothesis the explanation is this. The character of purple color is dependent upon two independent factors which, though sepa- rately heritable, are both required to produce it. Each of these white strains of sweet peas possesses one of these factors which can produce colored flowers only when united with its complement, a proof of which appeared upon interbreeding hybrid purples from such a cross. In short, the color purple depends upon the action of two complementary factors which follow the behavior of a dihybrid. (See Chap. VH, par. 10.) The gametic formulae for the two strains of white sweet peas used in this experiment are Cp and cP, respectively. C stands for a color factor without OLD TYPES AND NEW 161 which no color can appear, even though pigment for color may be present, and c is the absence of this factor, while P represents a purple pigment factor which only finds expression in the somatoplasm when C P FIG. 49. Diagram to illustrate the possible progeny from two hetero- zygous purple sweet peas according to data from Bateson. C, color factor (large circles) ; c, absence of C (small circles) ; P, pigment fac- tor (large crosses) ; p, absence of P (small crosses). In the zygotea within the checkerboard squares the gametic symbols are superimposed. taken together with the color factor C. The small letter p stands for the absence of the purple pigment factor. It will be seen that each of the white sweet peas whose formulae are given above lack one of the two essential factors for purple color. When the 162 GENETICS two are crossed, however, all the progeny are purple with the formula CcPp. These hybrid sweet peas upon gametic segregation theoretically produce four kinds of gametes, CP, Cp, cP, and cp which may combine as any other dihybrid in sixteen different ways. In this case, however, these combinations group themselves into only two phenotypes, purple and white, as indicated in the accompanying diagram (Fig. 49) in which C and c are represented by large and small circles respec- tively, while P and p are correspondingly indicated by large and small crosses. The gametic symbols are superimposed to form the zygotes. The theoretical expectation here shown was closely approximated in the actual results. It may be noted in passing that the seven kinds of white sweet peas resulting from the above cross, while phenotypically alike, that is, in the zygotic symbols of Figure 49, lacking either the large circle (color) or the large cross (pigment), belong to three distinct genotypes as follows : NUMBER OF ZTGOTE IN FIGURE 49 Without the pigment factor (large cross) Without the color factor (large circle) Without either pigment (large cross) or color (large circle) 6- 8-14 11 -12 -15 16 Among the purple peas are the following four geno- types : OLD TYPES AND NEW 163 NUMBER OP ZTOOTE IN FIGURE 49 3 4 Duplex for both color (large circle) and pigment (large cross) Duplex for color (large circle) but simplex for pigment (large cross) Simplex for color (large circle) but duplex for pigment (large cross) Simplex for both color (large circle) and pigment (large cross) 2-5 3-9 4-7-10-13 b. Castle's Agouti Guinea-pigs An illustration of a supplementary factor that acts only in conjunction with some other to bring about a modification, is the pattern factor demonstrated by Castle in his guinea-pigs. The wild gray, or "agouti," color of the hair of cer- tain guinea-pigs is due to the fact that pigment is distributed along the length of each hair in a definite pattern. The tip of a single hair is black followed by a band of yellow, while most of the proximal part which is more or less concealed by overlapping hairs is a leaden color. The distribution of pigment in such a pattern gives the characteristic gray, or agouti color to the coat when taken as a whole. Castle demonstrated the separate nature and be- havior of such a pattern factor when he discovered that it is transmitted independently of pigment, which is necessary to bring it to expression. He showed that upon crossing a solid black guinea-pig, unquestionably possessing pigment but no "pattern," with a white 164 GENETICS albino guinea-pig having no pigment, some of the offspring "reverted" to the ancestral agouti, or "pattern" type, thus proving that the pattern must be carried in this case by the white or albino guinea- pig as a factor independent of the color which is necessary for its expression. c. CiLenofs Spotted Mice Another instance of the interaction of supple- mentary factors is seen in the spotting of piebald mice. Cuenot discovered that such spotting is due to the absence of a uniformity factor which if present causes color to be uniformly distributed over the entire coat. Both of these independent factors, spotting and uniformity, are real and not imaginary, since they may be separately transmitted through albino animals in the same way as the pattern factor mentioned above, notwithstanding that in albinos both are hidden through the absence of pigment, upon the presence of which their visibility depends. Whenever piebald or spotted animals appear in a progeny derived originally from self-colored stock, it is evidently due to the absence of such a "uni- formity" factor as has just been described. Galton's theory of "participate inheritance" (page 121) is now satisfactorily explained as true al- ternative inheritance in which the mosaic appearance is caused by a Mendelian determiner, in this instance a spotting factor or, in other words, the absence of a factor for uniformity. OLD TYPES AND NEW 165 d. Miss Durham's Intensified Mice Miss Durham, in her work with mice, has demon- strated an intensifying factor, the absence of which she calls a diluting factor. The action of the former produces, as its name implies, intensity of color, while that of the latter serves to lessen the degree of intensity in which color appears. These factors of intensity and diluteness, it should be observed, do not in any way correspond to the duplex and simplex condition of a dominant color character, either of which would straightway appear if crossed with an albino. The factors of intensity and dilution of color are of an entirely different nature, as they have been proven to be indepen- dently transmissible through albinos where a color character could not appear because of the absence of pigment. The following illustration of this kind of sup- plementary factors taken from Miss Durham's experiments will serve to make the case clear. The symbols employed are : B = black pigment which masks brown, or chocolate. b = the absence of B, consequently chocolate. I = intensity factor. i = dilution factor or absence of intensity. C = a complementary color factor acting with P. P = a complementary pigment factor acting with C. BICP = black. BiCP = blue or maltese (dilute black). bICP = chocolate. biCP = silver-fawn (dilute chocolate). 166 GENETICS The crosses which were made are represented in the table below, in which the expectation according to the Mendelian dihybrid ratios is given in paren- theses after the actual results of each cross. BLACK BLUE CHOCO- SlLVER- (BICP) {BiCP) (blCP) (biCP) Black (BICP) X SUver-fawn (biCP) 9(9) 4(3) 3(3) 2(1) Blue (BiCP) X Chocolate (biCP) 42(45) 16(15) 14(15) 8(5) Blue (BiCP) X Silver-fawn (biCP) 0(0) 33(36) 0(0) 12(12) It will be seen that the actual results, even when such small totals are concerned, approximate very closely the expectation and are entirely consistent. e. Castle's Brown-eyed Yellow Guinea-pigs Recently Castle has shown that in guinea-pigs there is an independent factor for extension of pig- ment distinct from the uniformity factor already mentioned. The absence of this extension factor ("restriction ") is manifested by a lack of black or brown pigment everywhere except in the eyes and to a slight extent in the skin of the extremities, while the distribution of yellow is wholly unaffected by it. That such "extension" and "restriction" factors really exist, is proven in the following way : When a brown (chocolate) guinea-pig is crossed with an ordinary black-eyed yellow one, the young are all black pigmented, but by cross-breeding OLD TYPES AND NEW 167 these hybrid young four varieties are obtained in the next generation, viz., black, brown, black-eyed yellow, and brown-eyed yellow, the latter a variety unknown before Castle's experiment in breeding was made. For the sake of clearness the formation of the brown-eyed yellow is shown below in Figure 50. Black bE||BE Black 13 b e| BE Black JBe BE||Be Back. IO bE Black be||Be fllack-esed "Vino* BE bE Black Be bE Black 11 bE||bE Chocolate 15 be||bE Chocolate BE||be Black Be||be Black-e\|ed Yellow 12 bE||be Chocolate 16 be| |be 3rown-eyed Yellow FIG. 50. Diagram to illustrate the origin of a brown-eyed yellow guinea- pig from two heterozygous black parents based upon Castle's experi- ments. The factor for yellow (Y) is present in every gamete and is consequently duplex in every zygote but is hidden whenever the fac- tor B is present. B, black pigment hiding brown or chocolate ; 6, chocolate (absence of B) ; E, extension of B over the entire body hiding Y ; e, restriction of B to eyes alone thus exposing Y over the entire body. 168 GENETICS Symbols B = black pigment, hiding brown or chocolate. b = absence of B, or chocolate. Y = yellow pigment, hidden by B. E = extension of B over entire body, hiding F. e = restriction of B to eyes alone, thus exposing Y over the entire body. C = complementary color factor acting with P to produce color. P = complementary pigment factor acting with C to produce color. (The factors C and P may be omitted for the sake of simplicity, since they are present in each instance.) First Cross "Extended" chocolate (bEY) X black-eyed yellow (BeY) = black (BbEeYY). Second Cross When these cross-breds are mated with each other, they each form four kinds of gametes, BEY, BeY, bEY, and beY, which unite into sixteen theoretical genotypic possibilities, shown in Figure 50. These fall into four phenotypes, nine black (BEY), three black-eyed yellow (BeY), three chocolate (bEY), and one brown-eyed yellow (beY). The actual results in Castle's experiments gave all four kinds in close numerical agreement with this expectation. The action of extension and restriction factors is, therefore, plainly a case of Mendelian dihybridism in which two independent pairs of alternative char- acters are concerned. OLD TYPES AND NEW 169 6. RABBIT PHENOTYPES Perhaps no better application of the factor hy- pothesis may be found than the case of the color of rabbits. There are many varieties of rabbits so far as color is concerned, particularly among domesticated races. These varieties are now quite explainable by the factor hypothesis, as indicated in the table below. The sixteen kinds of rabbits there catalogued have THE FACTOR HYPOTHESIS APPLIED TO COLORS OF RABBITS CONSTANT FACTORS ALTERNATIVE FACTORS GAMETIC FORMULA PHENOTTPIC CHARACTER WHEN CROSSED WITH THE SAME KIND OP GAMETIC COMBINATION 1 2 3 4 5 6 7 8 Br B Y C E I U A AUIECiYBBr] Gray a aUIEC [YBBr] Black u A a AuIEC [YBBr] Gray spotted auIEC [YBBr] Black spotted i 1 U u A a A AUiEC [YBBr] Blue-gray aUiEC [YBBr] Blue (Maltese) AuiEC [YBBr] Blue-gray spotted a auiEC [YBBr] Blue spotted e U A AUIeC [YBBr] (Yellow (with white belly and tail) a aUIeC [YBBr] ( Sooty yellow (with yellow ( belly and tail) u U u A AuleC [YBBr] Yellow spotted a A auleC [YBBr] Sooty yellow spotted \ AVieC [YBBr] Cream a A aUieC [YBBr] Pale sooty yellow AuieC [YBBr] Cream spotted a auieC [YBBr] Pale sooty yellow spotted 170 GENETICS been obtained by Castle and other experimental breeders as well as many of the albino types that would double this list if c, or the factor for absence of color, should be substituted for C, the presence of color, in column 4 of the table on page 169. Explanation of Symbols in the Foregoing Table Br = a factor acting on C to produce brown pigmentation. B = a factor acting on C to produce black pigmentation. Y = a factor acting on C to produce yellow pigmentation. The three factors, F, B, Br, are present in every rabbit gamete and up to date have not been sepa- rable as independent unit characters, although they have been separated out in guinea-pigs and mice. There are no brown rabbits, because black always goes linked with brown covering the brown factor. Yellow rabbits result, as explained below, through the action of factor e. C = a common color factor necessary for the production of any pigment. It was discovered in 1903 by Cuenot. c = the absence of C which results in albinos, regardless of whatever pigment factors may be present. By changing C to c, sixteen kinds of albinos would be added to this catalogue, an addition of one phenotype and sixteen genotypes, all looking alike but breeding differently. E = a factor governing the extension of black and brown pigment, but not of yellow. 3 = the absence of extension or restriction of black and brown pigment to the eyes and the skin of the extremities only, while yellow remains extended and visible. Demonstrated by Castle in 1909. OLD TYPES AND NEW 171 7 = an intensity factor which determines the degree of pigmentation. It can be transmitted indepen- dently of C through an albino. Discovered by Bateson and Durham in 1906. i = the absence of intensity or dilution. Dilute black = blue. Dilute yellow = cream. Dilute gray = blue-gray. U = a factor for uniformity of pigmentation or "self- color" discovered by Cuenot in 1904. u = the absence of uniformity which results in spotting with white. A = a pattern factor for agouti, or wild gray color, which causes the brown and black pigments to be ex- cluded from certain portions of each hair, resulting in the gray coat. When present in the rabbit, it is also associated with white or lighter color on the under surfaces of the tail and belly. It was demonstrated by Castle in 1907. a = the absence of the agouti or pattern factor. 7. THE KINDS OF GRAY RABBITS Each of the apparent kinds of gray rabbits indicated in the foregoing table may be made up of various genotypes. For instance, there are thirty-two differ- ent genotypes, each of which is phenotypically a gray rabbit. The zygotic formula for each of these thirty- two possibilities is displayed in the next table, and it will be seen that these range all the way from rabbits homozygous in all their variable characters (No. 1) to those homozygous in none (No. 32). The progeny of these various types of gray rabbits when inbred will consequently vary from the pure 172 GENETICS THE KINDS OF GRAY RABBITS (Color only) GENOTYPE BYGOTIC FORMULA NUM- BER OF HET- EROZY- GOTIC FAC- TORS PHENOTYPES When inbred, these kinds are produced 1 AA UUIIEECC [YBBr] [YBBr] None X 2 AAUUIiEECc [YBBr][YBBr] One X X 3 AAUUIIEeCC [YBBr] [YBBr] One X X 4 AAUUIiEECC [YBBr][YBBr] One X X 5 AAUuIIEECC [YBBr] [YBBr] One X X 6 AaU UIIEECC [ YBBr] [ YBBr] One X X 7 AAUUIIEeCc [YBBr][YBBr] Two X X X 8 AAUUIiEECc [YBBr][YBBr] Two X X X 9 AA UuIIEECc [YBBr] [YBBr] Two X X X 10 AaUUIIEECc [YBBr] [YBBr] Two X X X 11 AAUUIiEeCC [YBBr][YBBr] Two X X X X 12 AA UuIIEeCC [YBBr] [YBBr] Two X X X X 13 AaU UIIEeCC [ YBBr] [ YBBr] Two X X X X 14 A A UuIiEECC [ YBBr] [ YBBr] Two X X X X 15 AaUUIiEECC [ YBBr] [ YBBr] Two X X X X 16 Aa UuIiEECC [ YBBr] [ YBBr] Two X X X X 17 AaUuIiEECC [ YBBr] [ YBBr] Three X X X X X X X X 18 Aa UuIIEeCC [ YBBr] [ YBBr] Three X X X X X X X X 19 AaUuIIEECc [ YBBr] [ YBBr] Three X X X X X 2!) AaUUIiEeCC [YBBr][YBBr] Three X X X X X X X X 21 AaU UliEECc [ YBBr] [ YBBr] Three X X X X X 22 AaUUIIEeCc [YBBr][YBBr] Three X X X X X 23 AAUuIiEeCC [YBBr][YBBr] Three X X X X X X X X 24 AAUuIIEeCc [YBBr][YBBr] Three X X X X X 25 A A UuIiEECc [ YBBr] [ YBBr] Three X X X X X 20 A A UUIiEeCc [YBBr] [ YBBr] Three X X X X X 27 A A UuIiEeCc [YBBr] [YBBr] Four X X X X X X X X X 28 AaUUIiEeCc [YBBr][YBBr] Four X X X X X X X X X 2!i AaUuIIEeCc [YBBr] [YBBr] Four X X X X X X X X X 30 AaUuIiEECc [ YBBr] [ YBBr] Four X X X X X X X X X 31 AaUuIiEeCC [YBBr][YBBr] Four X X X X X X X X X X X X X X X X f 32 Aa UuIiEeCc [ YBBr] [ YBBr] Five X X X X X X X X X X X X X X X X XI o 2 o a a a a a $ i y. o t o 5 3 a P3 s a c c c c o o_ Q 3 p fD - ir tj ff <, * w o o xs a < i i ?< 5 Jc 1 ^c : 1 J 1 r ft ft ? ! ?c ft f s e e 6 6 ci P* p f* S IQ "- <0 e 00 4 o e 000 4 o 6 a 00 3 * 000 5 4 060 0a 4 000 4 060 a 3 o a 0o 3 oe0 5 oee 0* 2 060 000 060 5 a 060 4 060 60 4 o0 060 4 060 60 3 o e 6-0 3 < e 060 3 060 060 2 000 o 5 4 60 o 4 4 e e 3 o e 3 a> e a> 3 oee 2 60 00 4 a 060 3 060 60 3 e a 3 e e 060 2 < a e e 2 060 060 2 (tee e a 1 a 000 * a 4 a e 3 60 o a 3 a 3 068 o a 2 4> a 2 e 2 oee 060 f oee 00 o e e 3 Pure red + (white = Hybrid FIG. 56. The result of crossing white wheat with trihybrid red wheat. somewhat lighter shade than the original pure red strain. When plants of this heterozygous sort are crossed together, they yield plants producing red-kerneled and white-kerneled wheats in the proportion of sixty- three to one. The sixty-three kinds of red wheats are BLENDING INHERITANCE 191 of varying degrees of redness and may be classified after the manner of fluctuating variations with the greatest number of kinds at the intermediate degree between pure red and pure white. (See Figure 57.) In order to test whether the sixty-four kinds of wheats produced in the second filial generation, as theoretically displayed in Figure 55, really contain separable, though indistin- guishable, determiners for red-kernel, Nilsson-Ehle produced families of the third filial generation by self-crossing plants of the second generation. It was to be expected that, if these hybrid wheats of the second generation carried one, two, three, or more determiners for a red kernel as the theoretical tables in Figures 55 and 57 demand, their progeny would be distributed with reference ,1 i j j FIG. 57. The distribution of the to the number of red- and six t y .f ou r possibilities in the F 2 white-kerneled individuals, generation when three similar , e ,, . determiners act together as a in the following ratios : trihybrid. # -h 4- 192 GENETICS 3 red to 1 white when 1 heterozygous determiner for red is present. 15 red to 1 white when 2 \ heterozygous deter- 63 red to 1 white when 3 > miners for red are All red to no white when 4 or more ) present. Among seventy-eight sample families of the third generation inbred to test this theoretical conclusion, the actual results were : 8 families giving the ratio of 3 red to 1 white. 15 families giving the ratio of 15 red to 1 white. 5 families giving the ratio of 63 red to 1 white. 50 families giving the ratio of all red to no white. It has been actually demonstrated therefore, in the case of this particular strain of wheat: (1) that the factors producing red kernel are several in number; (2) that they act independently of each other in heredity; (3) that these several independent factors segregate; and (4) that any one red factor acting alone produces a "red" result. The Nilsson-Ehle principle of cumulative determin- ers has been confirmed in America by East in a mas- terly series of breeding experiments upon maize. In connection with the Nilsson-Ehle principle, it will be seen that the possible number of intergrades between the two extremes increases rapidly as the number of duplicate determiners increases. Thus with six duplicate determiners for the same character present, the ratio of possible dominants to recessives in the second filial generation would be 4095 to 1. The reappearance of this single recessive among 4095 BLENDING INHERITANCE 193 dominants would be extremely unlikely, and it might easily be mistaken for a mutation or a freak. Appar- ent blends of all intermediate degrees, however, would be sure to appear. Yet these are not blends in the "melting-pot" sense at all, but strictly cases of Men- delian dominance and segregation. 9. THE APPLICATION OF THE NILSSON-EHLE EX- PLANATION TO THE CASE OF RABBIT EAR- LENGTH The so-called blending rabbit ears, along with other similar cases, can now be made to fall into line, as pointed out by Lang, with the Mendelian law of segregation. If we assume that the long ear of the lop rabbit has only three independent but equal determiners for excess length, the case becomes one of Mendelian trihybridism with cumulative factors, which works out like Nilsson-Ehle's red-kerneled wheat in the following manner: In general the average for full lop ear-length may be placed at 220 mm. and for the ordinary short- eared rabbit 1 at 100 mm. The difference, or the ex- cess length of the lop ear, is 120 mm., which, according to the trihybrid formula, corresponds to the six doses of the character symbolized in the upper left-hand square in Figure 55 by six large screw heads, three 1 Not the Belgian hare, as cited in the illustration given in Figure 52. The Belgian hare has typically a somewhat longer ear than the ordinary short-eared rabbit. O 194 GENETICS coming from each parent respectively. If all of these independent determiners are equal as regards excess ear-length, each factor would represent an excess of 20 mm. above the normal ear-length found in short- eared rabbits, that is, 220 mm. - 100 mm. 6 = 20 mm. When according to this computation a lop (20 mm. X 6 + 100 mm. = 220 mm.) and a pure short- eared rabbit (20 mm. X + 100 mm. = 100 mm.) are crossed, if imperfect dominance occurs, which is a very common phenomenon, it is true that the offspring might present a "blended" appear- ance. If now these cross-breds of the first gen- eration prove to be trihybrids with respect to excess ear-length, there would be sixty-four possibilities in their progeny segregating out just as in the red- kerneled wheat. These possibilities would be arranged in the fol- lowing frequencies: NUMBER OF EXCESS EAR- LENGTH DETERMINERS NUMBER OP CASES OCCUH- RINQ OUT OP 64 TOTAL LENGTH IN MILLI- METERS OP EARS RESULTING 6 1 220 5 6 200 4 15 180 3 20 160 2 15 140 1 6 120 1 100 BLENDING INHERITANCE 195 Since the average litter among rabbits is about five, the chances that these five rabbits will breed true to their hybrid parents and form a perfect blend between their grandparents is 20 out of 64, while the chance of their being like either grand- parent is only one out of 64. It should be noted further that 50 out of 64, or 77 per cent, of these hybrids of the second filial gen- eration would have an ear-length between 140 and 180, thus approximating a "blend" closely enough to be so classified upon a casual inspection. Moreover, if it should be found that excessive ear-length in rabbits is due to more than three dupli- cate determiners, the possibilities of getting anything but an apparent blend would be much decreased. The fact, furthermore, that the fractional ear- lengths of the hybrid rabbits in Castle's experiments bred approximately true in the second and subse- quent filial generations, may also be explained by the Nilsson-Ehle hypothesis. For example, half lop lengths, according to this explanation, are those with three doses of the deter- miner for excess ear-length. It follows that the progeny of two rabbits each carrying three doses of a determiner will likewise, after the reduction during the maturation of the germ-cells, have three doses fa I o \ of the determiner ( = 3 ). \ 2 J It would be interesting to breed rabbits having ears of one eighth lop length in which, according to the foregoing hypothesis, there would presumably be 196 GENETICS present only a single determiner for excess ear-length, with ordinary short-eared rabbits having no excess ear-length, in order to see if the expected Mendelian three-to-one proportion for a monohybrid would ap- pear in the progeny. 10. HUMAN SKIN COLOR Finally, although accurate published data are wanting, it is probably true that skin color in all kinds of hybrids resulting from crosses between negroes and whites is not a case of blending inherit- ance, as commonly supposed, but rather of true. Mendelian segregation. In fact, there is frequently visible evidence that segregation does occur, as shown by many authentic instances where the offspring of diversely colored parents produce children with skin color of different shades. If human families included hundreds of offspring in a single generation instead of the usual number, the problem of skin color in man could doubtless be quickly solved since ratios could then be obtained large enough to reveal the underlying laws of inher- itance. CHAPTER X THE DETERMINATION OF SEX 1. SPECULATIONS, ANCIENT AND MODERN FROM the earliest times the desirability of con- trolling the sex of an unborn child, in particular instances at least, has seemed very great. Likewise the wish to be able to predetermine sex among do- mesticated animals has made breeders quick to grasp at every clue that promised success. There has been no want of speculations concerning the determination of sex. J. Arthur Thomson, who with Professor Geddes wrote "The Evolution of Sex" in 1889, says : "The number of speculations as to the nature of sex has been well-nigh doubled since Dreylincourt, in the eighteenth century, brought together 262 'groundless hypotheses' and since Blumenbach caustically remarked that nothing was more certain than that Dreylincourt's own theory formed the 263rd. Subsequent investigators have long ago added Blumenbach's theory of 'Bildungs- trieb' or formative impulse, to the list." It maybe added in passing that the hypothesis of the deter- minative action of external factors upon developing germ-cells which Geddes and Thomson elaborated in the book just referred to, has, in its turn, accord- ing to most biologists, joined the long roll. 197 198 GENETICS Hippocrates thought that sex of the offspring depends upon the relative "vigor" of the parents, while Sadler (1830) concluded that the relative ages of the two parents is the determining factor. Other writers, on the contrary, have thought that the age of the mother at the time of childbirth deter- mines the sex of the offspring, and Thury (1863), in the days before the facts of maturation were known, ascribed the determinative factor to the relative degree of "ripeness" of the egg when fertilized. It was once assumed also that the right ovary or the right testicle is the seat of one sex and the left ovary or left testicle of the other. Galen, who did the biological thinking for several centuries of mankind, asserted that the right side of the body, "being warmer" than the left, consequently produces males. Schenk cites a most amazing bit of folk-lore to the effect that: "In Servia if a man has a stye on his eyelid he comes to the conclusion that his aunt is pregnant. If the stye is on the upper eyelid, the child will be a male ; if on the lower, a female." Modern theories of sex determination, like the earlier speculations, may be resolved into two groups, namely, those which depend upon controllable ex- ternal or environmental factors such as food, climate, chemical dosage and will power, and those which de- pend upon internal factors at present beyond control. 2. THE NUTRITION THEORY Of external factors which may exert a moulding influence upon the sex of the offspring, nutrition is THE DETERMINATION OF SEX 199 possibly the most potent. This factor may be conceived to act either upon the parent previous to the maturation of the germ-cells, upon the germ- cells themselves, or upon some susceptible embryonic stage of the life cycle subsequent to that of the fer- tilized egg. It has been suggested that since the egg is char- acterized by possibly a more advanced metabolic condition than the sperm due to the presence of the nutritive yolk, consequently the more yolk or nutri- tion there is, the more f emaleness will characterize the egg. In other words, f emaleness is a nutritive condi- tion associated in the egg with the presence of yolk. A generation ago Professor Schenk of Vienna, by controlling the nitrogenous diet of certain royal prospective mothers, gained a soothsayer's reputa- tion as a prophet of sex which was based upon several correct predictions. Of course, any prediction of sex is bound to turn out correct in 50 per cent of the cases, regardless of what it is based upon, since in man the two sexes are approximately equal in numbers. Adherents of all sorts of theories, therefore, have always been able to produce considerable "evidence" to sub- stantiate their speculations, however crude the latter have been. Statisticians have pointed out that in times of unusual hardship, like famine or war, when the amount of available nutrition for pregnant mothers is presumably reduced, there seems to be a prepon- derance of males born. 200 GENETICS A series of nutrition experiments upon frogs per- formed by Born ('81), Pfliiger ('82), and Yung ('85) showed that the percentage of female offspring, which normally is slightly over fifty, could be changed to over 90 per cent by regulating the food supplied to the mother before the egg-laying period. Cuenot and King, however, working independently, repeated these experiments with great care, taking into account all the eggs that were laid and not simply the ones that developed, and both obtained negative results. They concluded, therefore, that the high percentage of female tadpoles appearing in the initial experi- ments was due to a greater mortality among the males and not to the transformation of possible males into females. There seems to be no doubt that nutrition may affect the percentage of those which reach maturity. If one sex requires a greater amount of nutrition than the other to carry out successfully the more strenuous metabolic changes in its life-cycle, then unequal percentages between the sexes of the sur- vivors resulting from modified nutrition do not in any way help to solve the problem of determining the sex of the individual. In other words, the elim- ination of one sex through modified nutrition does not "determine" the other sex. 3. THE STATISTICAL STUDY OF SEX From statistical sources it has been ascertained that ordinarily there is produced a practical equality in the numbers of the two sexes. THE DETERMINATION OF SEX 201 Oesterleben in Europe summarized the data for nearly sixty million human births and found that an average of 106 males are born to every 100 fe- males. According to various authorities, the relative num- ber of males per 100 females is given for horses as 99, for cattle 94, and poultry 95, while in pigs, rabbits, pigeons, and greyhounds the corresponding number of males is slightly over 100. This practical equality of the sexes in all sorts of natural environments indicates the improbability of the assumption that external conditions determine sex. 4. MONOCHORIAL TWINS There are two kinds of twins, namely, ordinary twins, which come from two separately fertilized eggs each inclosed in its own chorion, and "identical twins, " that have their origin in one egg which is in- closed in one chorion. Of the former, something like 30 per cent in man are reported as being of two sexes, thus showing that it is neither nutrition nor envi- ronment which determines sex. Usually when twins are of the same sex, they exhibit as great a range of difference in mental and physical traits as do ordinary children of the same fraternity born at different times, but occasionally "identical twins" are born, and such monochorial twins are always of the same sex. This is evidence that sex, like other somatic characters, is determined in the germplasm at the time of fertilization. 202 GENETICS Similarly, in the chalcid fly, Ageniapsis, a chain of embryos is formed from a single egg, and these, ac- cording to Marschal, are all of the same sex. Newman and Patterson also have shown that in the armadillo, Tatusia, there are customarily produced four young within a single chorion, all of which are of the same sex. These facts point toward the conclusion that the determination of sex takes place at the time of fer- tilization. 5. SELECTIVE FERTILIZATION Within the last ten years considerable evidence has been collected in support of the supposition that sex is a Mendelian character. Mendel himself, without elaborating this idea into a definite hypothe- sis, suggested the probability that sex is a heritable character behaving in the same way as other herit- able characters. In 1903 Castle published a paper l in which a tentative explanation, since abandoned, of the phe- nomenon of sex determination was advanced, based upon three assumptions : first, that all germ-cells are heterozygous for sex and, therefore, upon maturation there are formed both male and female eggs as well as male and female sperms ; second, that in fertili- zation the gametes always unite with their opposites so far as sex is concerned and never with their like, with the result that each fertilized egg must carry Castle, W. E., "The Heredity of Sex." Bull. Mus. Comp. Zool., Harvard, Vol. XL, No. 4, 1903. THE DETERMINATION OF SEX 203 determiners for both sexes and be heterozygous, as indicated in Figure 58 ; and third, that the character of sex follows the law of alternative dominance, ac- cording to which in the male offspring the male determiner dominates M(F), while in the female the female dominates (M}F. This hypothesis is simply an attempt to explain the numerical equality of the sexes, and also the fact that the determiner for the opposite sex may be car- MALC GAMETES ZYGOTES M(F) M(M) (F)F F(M) MALE -DO NOT OCCUR FEMALE FIG. 58. Diagram to show Castle's 1903 theory of the heredity of sex. ried by either parent, but it leaves unanswered the question of what causes "selective fertilization" and "alternative dominance." There appears to be some evidence that selective fertilization, which was assumed in Castle's 1903 theory, may actually occur under certain circum- stances. For example, homozygous or pure yellow mice, that is, mice with a duplex determiner for yellow color, are not known. In breeding, all kinds of yellow mice behave as if heterozygous or simplex with respect to yellow color, for when any two yellow mice are bred together, they produce a certain per- centage of recessives which would not happen if they 204 GENETICS were pure yellow. In a Mendelian monohybrid cross, as has been previously pointed out, the expectation is that in the second generation one fourth of the offspring will be recessives (DR XDR = DD + 2DR + RR) , but when yellow mice are bred together, the percentage of recessives approximates one third instead of one fourth. This apparent exception to the Mendelian ratio finds an explanation, however, when it is assumed that selec- tive fertilization takes place in such a cross, and thus, since a D gamete never unites with another D gamete, but always with its opposite, R, pure yellow mice are unknown. This supposition is further supported by the fact that the litters of young from yellow mice are, on an average, only three fourths as large as normal litters of mice, which is exactly what would be expected if one fourth of the possible gametic combinations (DD) fail to produce offspring. Castle's tentative explanation of the determina- tion of sex at least breaks away from the old concep- tion that the sperm-cell produces male offspring and the egg-cell, females. It agrees, too, with Darwin's idea that both sexes are present in each individual with one sex latent. In certain parthenogenetic rotifers, aphids and daphnids, both sexes are plainly present in the female, since two kinds of easily dis- tinguishable eggs are produced, one of which develops into males and the other into females without fer- tilization or any kind of a union with a sperm-cell. THE DETERMINATION OF SEX 205 TYPE I n ZYGOTES (Female) 9O* (Male) Cfcf (Female^ (Male) GAMETES $ + rf d 1 {Finale) (Male) 99 Vcf bmfe) (hate (Hate) FIG. 69. Diagram to show the neo-Men- delian theory of the heredity of sex, using sex symbols. 6. THE NEO-MENDELIAN THEORY OF SEX Correns (1906) avoids the difficulties of alternative dominance, which Castle's hypothesis offers, by sup- posing that one par- ent only is hetero- zygous with respect to sex, and this sup- position is becom- ing more and more probable as evidence accumulates. Ac- cording to this idea, there are two types of cases, one when the female is the heterozygous parent and the other when the male is the hetero- zygous parent, as represented in Figure 59. The formulae for these types may be expressed in the nomenclature of the presence and absence theory, as follows (Fig. 60), in which the sym- bol x represents the female determiner in the heterozygous case of type I, and xx the female de- terminer when the male is the hetero- zygous parent. The formulae may be still further modified, accord- ing to Morgan, for the satisfaction of those who ob- TYPE ZYGOTES GAMETES I (Female) x O (Male) o O X -1- O -f- xo oo (Female) (Male) n (Female) x X (Male) x O X + X X + O XX XO (Female) (Male) FIG. 60. Diagram to show the neo-Men- delian theory of the heredity of sex accord- ing to the presence and absence hypothesis. 206 GENETICS ject to regarding the male factor as nothing positive, but simply the absence of femaleness, by assuming that a universal factor of maleness (m) is present in all cases, as shown in Figure 61. Thus in type I of this scheme it is only when the dominant female factor F is entirely absent that male- ness becomes expressed in the somatoplasm, while in type II it is neces- sary to have a double dose of the factor F in order to produce a female, since a single dose results in a male. All of these three theoretical schemes agree in assuming that one sex is hetero- zygous, while the other is homozygous and that femaleness is the result of an added factor in excess of maleness. The evidence for these conclusions has been ob- tained chiefly from four sources : first, from a mi- croscopical examination of the germ-cells ; second, from castration and regeneration experiments ; third, from the results of hybridization in "sex-limited inheritance"; and fourth, from the behavior of hermaphrodites in heredity. TYPE ZYGOTES GAMETES i (Female) Frofm [Male) fmfm Fro 4- fm fro 4- fm Fmfm fmf Femate) (Mate) n (Female) FmFm Mole) Fmfm Fm -f Fm Fm -|- fm FmFm Fmfm IFemate) (Male) FIG. 61. Diagram to show the neo-Men- delian theory of the heredity of sex with Morgan's modification, making male- ness (TO) present as a positive character in every gamete. THE DETERMINATION OF SEX a. Microscopical Evidence 207 A, 1. The "X" Chromosome In 1891 Henking called attention to the presence of two kinds of spermatozoa in the firefly, Pyrrho- coris, and later McClung (1901), in studying the spermatogenesis of the grasshopper, discovered a similar phenomenon with respect to the chromosomes of its spermatozoa. Soon after, Stevens and Wilson FIG. 62. Diagram to show how numerical equality of the sexes results when one parent is homozygous (the female in this instance) and the other is heterozygous for the sex character. working independently on various species of insects, and Boveri, on sea-urchins, found that when the male is characterized by two kinds of sperm-cells, one of which has an "extra" chromosome (the so- called "accessory" or "z" chromosome), while the [other does not, the female of the same species, upon [maturation of the eggs, produces mature eggs, all of which possess one "z" chromosome. The result jof this heterozygous condition of the male and homo- Izygous condition of the female with respect to the x chromosome is the theoretical equality of the sexes among the individuals formed by their union, as 208 GENETICS shown in Figure 62 or in type II of Figures 59, 60 and 61. It will be seen that when a male gamete bearing an x chromosome unites with a female gamete also bearing an x chromosome, the outcome is a fertilized egg containing xx chromosomes. Such an egg is con- sequently homozygous for sex, and will develop into a female individual. In the same way when a male gamete lacking an x chromosome, as half the gametes derived from a heterozygote do, unites with a female gamete bearing an x chromosome, as all gametes from a homozygote must do, then the fertilized egg will be heterozygous, carrying only one x chromosome, and will develop into a male individual. The chromatin difference between the two sexes may be qualitative, as Wilson holds, or quantitative, as Morgan assumes, but in either case it seems cer- tain that, with difference in sex, there is invariably associated a definite difference in the character of the chromosomes present in the germ-cells. These conclusions have been abundantly con- firmed in various species by a large number of inde- pendent workers, and are now well established as a part of biological science. In fact, it is not at all unusual to find the technical confirmation of the x chromosome theory given as a part of the routine class work in university courses. A t 2. Various Forms of X Chromosomes The extra chromosome in different species may assume various forms or degrees of complexity. It THE DETERMINATION OF SEX 209 may be either single or multiple. It may be paired before maturation with its absence, or with an unlike ("?/") chromosome. It may be linked inseparably with some one of the ordinary chromosomes (auto- somes), or resemble the autosomes so closely that its presence can only be assumed from analogy with other cases, and not definitely determined at all. In all of these cases, however, there is one point of likeness, and that is that there always seems to be additional chromatin material associated with the female sex. The reason for this may lie in the more highly metabolic requirements of the female, who must produce yolk or provide in some way for the main- tenance of the young in addition to furnishing half of the germinal heritage. In the microscopical evidence on this point there is one apparent exception to the rule that females are homozygous and males heterozygous with respect to sex. Baltzer (1910) found that in one of the sea- urchins an extra sex chromosome is associated with the female sex, so that two kinds of mature eggs are produced upon maturation ' and only one kind of sperm-cells. In other words, in this case the female is heterozygous for sex and the male homozygous, instead of the reverse which is true for all other forms thus far microscopically investigated. Such cases as this of the sea-urchin are theoreti- cally provided for in the formulae under type I given above in Figures 59, 60 and 61. 210 GENETICS A, 3. Sex Chromosomes in Parthenogenesis The behavior of the chromosomes in cases of parthenogenesis, where the union of an egg-cell and a sperm-cell are not necessary for the production of a new individual, throws additional light upon the relation between chromosomes and sex determi- nation. For instance, among the social Hymenoptera, bees, ants, wasps, etc., the "queen" produces eggs which upon maturation, if unfertilized, develop into males or drones, all of whose cells contain a reduced amount of chromatin (Fig. 63). It is only when sexual repro- duction occurs through the union of a mature egg- cell and a mature sperm-cell or spermatozoan, that the full complement of chromatin is restored to the fertilized egg and females are again produced. Castle says : "In all known cases of parthenogenesis the female is in the duplex (2 n) condition, and the male is in the simplex (n), or partially duplex (2 n 1 condition. The female in all cases has the greater chromatin content." b. Castration and Regeneration Experiments Certain characters which are known as "secondary sexual characters," such as the ornamental plumage in male birds, the beard in man or the sting in worker bees, are often associated with a definite sex. When an individual is castrated, it is quite common not only for these peculiar secondary sexual characters to dis- appear, but also for the secondary sexual characters of THE DETERMINATION OF SEX 211 9 SOMA (Queen) OOCYTC ( J cfSQMA (Drone) U RUDIMENTARY Z."-* SPERMATOCYTE SPERMATOCYTE I *J POLAR BODY 2 n - d OocvTt \~ POLAH BOPV i** POLAR BODY MATUftt E6& SPERM CELLS FERTILIZCD EGG FIG. 63. Diagram of the heredity of sex in bees, ants and wasps. The outline chromosomes represent sample somatic chromosomes. The solid black chromosomes stand for sex. The female has two sex chromosomes while the male has but one. the opposite sex to develop to a certain degree in their stead. This indicates that the determiners for sex are intimately associated with those for the sec- 212 GENETICS ondary sexual characters, and also that the determin- ers for the opposite sex are often present in a latent condition, or, in other words, that the organism, either male or female, is heterozygous with respect to sex. If a female of the annelid worm Ophiotrocha, for example, is cut in half, it is effectually castrated, be- Male Parasitical^ castrated male Female FIG. 64. The crab, Inachus, parasitized by the cirripede, Sacculina. Evidence that a Mendelian sex determiner is correlated with " sec- ondary sexual characters " and that the male is heterozygous for sex while the female is homozygous. After Smith. cause the ovaries are in the posterior part of the body. It has the power of regeneration, however, but when a new posterior part is formed, it contains, not female, but male reproductive organs. The worm is, there- fore, now a male, as shown by the presence of testes instead of ovaries, proving that it was originally heterozygous with respect to sex. carrying one sex latent. THE DETERMINATION OF SEX 213 According to Smith, parasitic castration is performed on the crab Inachus, which is found in the Bay of Naples, by a cirripede, Sacculina. The male crab of this species has one large claw and a narrow abdo- men, while the female has no large claw, but a broad abdomen. When Sacculina parasitizes the female, the secondary sexual characters of the female are stunted, but not materially changed. When, on the contrary, the male is parasitized, it not only loses its distinctive large claw in subsequent molts, but it also takes on the broad abdomen of the female (Fig. 64). This apparent anomaly is quite explainable upon the as- sumption that the female is homozygous for sex and the accompanying secondary sexual characters, while the male is heterozygous. When maleness is destroyed in the male by the castrating parasite, therefore, the femaleness that is latent in this sex becomes manifest through the appearance of female secondary sexual characters ; but when the female is castrated, no other secondary sexual characters than those already present make their appearance, since only femaleness is present in the homozygous female sex. c. Sex-limited Inheritance Additional evidence that sex is a character depend- ing upon determiners which behave in Mendelian fashion is furnished by what is called sex-limited inheritance. There are certain characters known as sex-limited characters that are in no sense to be con- fused with secondary sexual characters which appear to be always linked with the determiner for either one 214 GENETICS sex or the other. They are, therefore, well described by the term "sex-limited." (1) Color-blindness This phenomenon may be illustrated by the in- heritance of human color-blindness, a character which appears to be linked with the determiner for sex. It requires a duplex, or homozygous, dose of the deter- miner for color-blindness to produce a color-blind female, while only a simplex, or heterozygous, dose is d 1 FIG. 65. General diagram for sex-limited inheritance. The underscored symbol (21) represents a sex determiner with some other character (as color-blindness) linked with it. needed to produce a color-blind male. These facts agree perfectly with the idea that the female is homo- zygous and the male heterozygous with respect to THE DETERMINATION OF SEX 215 sex, and that the factor for color-blindness is linked with the determiner for sex. Sex-limited inheritance, as shown in this case, may be illustrated by the dia- gram on the opposite page (Fig. 65) in which, for the sake of simplicity, only sex chromosomes and the de- terminers for color-blindness are represented. Under- scored x. represents a color-blind determiner linked to a sex chromosome. From this diagram, which agrees substantially with the facts, it is apparent that a color-blind male mated to a normal female will produce no color-blind offspring, although the females will be "carriers" of color-blindness, that is, will possess the factor in simplex form and will, therefore, carry it for the female in a latent condition. The sons of such a mating having a normal mother and a color-blind father will be absolutely free from the defect and cannot produce color-blindness in any of their offspring when mated with a normal strain. If, however, the "carrier" daughters from such a parentage, who are genotypically heterozygous for color-blindness but phenotypically normal, mate with normal individuals, the expectation is that one half of the sons, and none of the daughters will be color- blind, but that one half of these daughters will carry the color-blind determiner in simplex form, that is, in a condition ineffective for producing color-blindness in female individuals. All of the various possibilities in the inheritance of color-blindness according to the sex-limited interpre- tation are indicated in the following table : 216 GENETICS PABENTS EXPECTED OFFSPRING Normal 9 Color-blind $ Color-blind 9 Carrier Normal Carrier color-blind 2 normal 5 carrier normal Color-blind Normal Normal Carrier Color-blind Color-blind Color-blind Color-blind Color-blind Carrier ^ color-blind % normal color-blind carrier (2) The English Currant-worm A famous case of sex-limited inheritance is that of the English currant-worm, Abraxas, which occurs in two varieties, viz., Abraxas grossulariata and Abraxas FIG. 66. Abraxas grossulariata, the English currant-moth, and (on the right) its paler lacticolor variety. From Punnett's " Mendelism." lacticolor (Fig. 66). The lighter-colored lacticolor is recessive to the darker-colored grossulariata variety and has been found in nature associated only with the female sex. THE DETERMINATION OF SEX 217 Doncaster and Raynor, in 1908, published the results of various crosses between these two varieties which demonstrate clearly that sex is a Mendelian character and that, in this instance, maleness is homozygous and femaleness heterozygous with the determiner * KEY TO SYMBOLS PHENOTYPE GROSS.C? GROSS, c? LACT.6 1 lOnstitution with respect lo the GROSSULARIATA Duplex Simplex Nulliplex Simplex Nulliplex .ENOTYPE 001 Ooi Ooi Ooi Qoi floi 00i So Ooi Oo GAMETES FIG. 67. Key to the symbols employed in Figures 68-71. The outline symbols represent samples of the autosomes or somatic chromosomes. The black symbols stand for the "extra" or sex chromosomes. O above a black symbol indicates the grossulariata factor linked with a sex chromosome. The variety lacticolor occurs whenever the grossu- lariata factor is absent. for maleness linked with the factor producing the variety grossulariata. A study of Figures 67-71 will make this case clear. Outline symbols represent ordinary chromosomes or autosomes, several of which are omitted for sake of clearness. The black sym- bols represent sex chromosomes. The letter G placed 218 GENETICS above a black symbol represents the grossulariata factor linked with a sex chromosome. The variety lacticolor occurs whenever the factor for grossulariata is absent. In this case two sex determiners are neces- sary to produce a male, and only one to produce a female. In the following theoretical diagrams the actual number of offspring obtained by Doncaster and Raynor in each cross is indicated outside the circles that represent the zygotes, and the parenthetical numbers refer to the five kinds of individuals cata- logued in Figure 67. In the first cross (Fig. 68) where a lacticolor female (5) and a grossulariata male (1) were bred together, the entire progeny was grossulariata in character with an approximate equality between the sexes, that is, 45 males (2) to 50 females (4). When these hybrid grossulariata individuals, (2) and (4), were mated with each other in Cross 2 (Fig. 69), the character of grossulariata appeared again in both sexes, (1), (2), and (4), while the character lacticolor was confined as usual to females alone (5). It was only when grossulariata hybrid males (2) were crossed back to lacticolor recessive females (5) in Cross 3 (Fig. 70) that individuals of both varieties and both sexes appeared, (2), (3), (5), (4), in practically the expected equal numbers, namely, 63, 65, 70, 62. The lacticolor male (3) obtained by bringing together the two sex determiners necessary for maleness, each of which had been dissociated through the foregoing crosses from the sex-limited grossulariata factor, was en- tirely new to science, never having been found in nature. THE DETERMINATION OF SEX 219 CRO ss 1 (0 GROSS. c? GAMETES ZYGOTES (2) FIG. 68. The formation of heterozygous grossulariata individuals, both male and female, by crossing pure grossulariata males with lacticolor females. CRO ss 2. (2) GROSS, d" GROSS.C? GROSS.C? (/) (2) FIG. 69. The cross-breeding of heterozygous grossulariata individuals. LACT. 9 GROSS. (5) (4) 220 GENETICS Finally, when these newly made lacticolor males (3) were crossed with heterozygous grossulariata females (4) (Fig. 71), the proportion of sexes was approxi- mately equal, as expected, that is, 145 males to 130 females, but all of the males were of the heterozygous grossulariata type (2) and all of the females of the re- cessive lacticolor type (5), showing a return to the sex- limited condition. All of these curious results find a satisfactory and complete explanation in the assump- tion, first, that sex is a Mendelian character carrying two determiners for maleness and one for femaleness ; and, second, that the determiner for the character of grossulariata when present is always linked to the sex determiner. . ; This case is of particular interest, since it agrees with the microscopical evidence already referred to in connection with the chromosomes of Baltzer's sea- urchins, in which the male was likewise homozygous and the female heterozygous with respect to sex. The chromosomes of Abraxas present certain techni- cal difficulties which at present have not been over- come, so that we do not yet know whether the evi- dence of the heterozygous character of one sex and the homozygous character of the other, obtained from the breeding experiments of Doncaster and Raynor w will be confirmed upon a microscopic examination of the chromosomes in the germ-cells. (3) The Behavior of Hermaphrodites in Heredity Certain plants occur in monoecious form, that is, as hermaphrodites, and also in dioecious form, that is, with THE DETERMINATION OF SEX 221 CROSS 3 GRoss.d* LACT.C? LACT. o GROSS Q (5) (4) FIG. 70. Heterozygous grossulariata male crossed with lacticolor female. One fourth of the progeny are lacticolor MALE, not known to occur in nature. CROSS 4 (4) GROSS. FIG. 71. Back cross of lacticolor male with grossulariata female produc- ing the original sex-limited condition in which all the females are of the lacticolor type. Data for Figures 68-71 from Doncaster and Raynor. 222 GENETICS the sexes on separate plants. Among such dimorphic plants, Bryonia in particular has been investigated by Correns and Lychnis by Shull. Without describing the crosses made in their experiments in detail, it may be stated that when dioecious types are recipro- cally crossed with hermaphroditic forms, the result- ing progeny indicate plainly that one sex is homozy- gous while the other is heterozygous with respect to the sex character. This confirmatory evidence is quite in line with that already brought forward that sex is a Mendelian character the determiners of which are carried in the germplasm. 7. CONCLUSION The evidence thus far obtainable from all sources points to the conclusion that sex is unalterably fixed at the time the egg is fertilized, by definite deter- miners which act in the same way as other Mendelian determiners. Dr. Shull, whose exhaustive studies in sex determination place him in the front rank as an authority on the subject, makes this conservative statement: "Nearly all the recent investigations indicate that sex is at least predominantly dependent upon the genotypic nature of the individual." If this is so, while it furnishes the best of confirma- tory evidence in support of Mendel's law, it shows that it is not possible for man to predetermine the sex of his offspring, which he has long hoped to be able to do. The following quotation from Castle may suitably close this chapter: "Negative as are the results of our study of sex control, they are perhaps THE DETERMINATION OF SEX 223 not wholly without practical value. It is something to know our limitations. We may thus save time from useless attempts at controlling what is un- controllable and devote it to more profitable employ- ments." CHAPTER XI 1. THE APPLICATION or GENETICS TO MAN HUMAN civilization goes hand in hand with the degree of successful interference which man exerts upon the natural forces surrounding him. Primitive man was overwhelmed and outmastered by his environment, but civilized man harnesses nature to do his will. Savages are not proficient in the arts of cultivating plants and domesticating animals, while these are the very things upon which human prog- ress fundamentally depends. The degree of civiliza- tion of any people is closely correlated with the degree of their success in exercising a conquering control over plants and animals. Any knowledge of the laws of heredity, therefore, as applied by man, either directly to himself or indirectly to animals and plants, is a distinct contribution to human progress. In 1900 the National Association of British and Irish Millers, as Kellicott points out, being dissatis- fied with the quality and quantity of the annual wheat yield, engaged Professor Biffen to apply his knowledge of heredity to the practical problem of improving their wheat crop. The characters desired were a short full head, beardlessness, high gluten 224 THE APPLICATION TO MAN 225 content, immunity to rust, strong supporting straw, and a high yield per acre. In the short time that has elapsed, Professor Biffen has succeeded in pro- ducing strains of wheat that combine all these de- sirable characters to a remarkable degree. Such an immediate result would not have been pos- sible before 1900, when the rediscovery of Mendel's law revolutionized man's knowledge of the action of heredity in nature. This same knowledge which has made possible the improvement of wheat may be applied to the breed- ing of man, for there is no reasonable doubt that man belongs in the same evolutionary series with all other animals, as Darwin showed, and is consequently subject to the same natural laws to a considerable degree. It must be admitted that thus far in the progress of civilization more attention has been directed to the scientific breeding of animals and plants, little as that has been, than to the scientific breeding of man. Let us hope that the future will have a dif- ferent story to tell ! 2. MODIFYING FACTORS IN THE CASE OF MAN There are certain qualifying factors which make the problems of genetics somewhat different in the case of man than of other organisms. For example, mankind has come to be partially exempt from some of the natural laws that affect other organisms. Thus with respect to the workings of natural selection man is partially under "grace" Q 226 GENETICS rather than "law." Nature no longer " selects * good eyes in man by long, patient, and devious processes when poor eyes are made good almost in- stantly by a visit to the oculist. She has long since given up providing natural weapons of defense for those who have the wits to supply themselves more efficiently with artificial means of self-preservation, and she no longer attempts to improve the natural powers of locomotion of those who are able to tame a horse to ride upon, or who build steamships, rail- roads, automobiles and aeroplanes, thus accom- plishing at once what would require ages at least to evolve. Neither does the law of the survival of the fittest in its original sense apply equally to man and to other organisms. Human society to-day protects its unfit in hospitals, asylums, and through various philan- thropies, while physicians devote themselves to the art of prolonging life beyond the period of usefulness. We do not desire these results of our modern civili- zation to be otherwise, but the fact remains that some of the most inflexible and universal "natural laws" are ineffective in the case of man, and it is profitable to bear this in mind when applying the laws of ge- netics to man. The laboratory for human heredity is the wide world, but it is obvious that the experimental method which has proven so effective in studying the heredity of animals and plants is impracticable in the case of man. The consideration of human heredity, there- fore, must always be largely from the statistical side, THE APPLICATION TO MAN 227 consisting in an analysis of experiments already per- formed rather than in initiating new experiments. Such institutions as insane asylums, prisons, sanitariums, and homes for the unfortunate are excellent foci for studying certain phases of human heredity, because they are simply convenient places where the results of similar experiments in genetics have been brought together. 3. EXPERIMENTS IN HUMAN HEREDITY a. The Jukes A classic example of an experiment in human hered- ity which has been partially analyzed by the statisti- cal method is that furnished by Dugdale in 1877 in the case of "Max Jukes" and his descendants. It includes over one thousand individuals, the origin of all of whom has been traced back to a shiftless, illit- erate, and intemperate backwoodsman who started his experiment in heredity in western New York when it was yet an unsettled wilderness. In 1877 the histories of 540 of this man's progeny were known, and that of most of the others was partly known. About one third of this degenerate strain died in infancy, 310 individuals were paupers who all together spent a total of 2300 years in alms- houses, while 440 were physical wrecks. In addition to this, over one half of the female descendants were prostitutes, and 130 individuals were convicted crim- inals, including 7 murderers. Not one of the entire family had a common school education, although 228 GENETICS the children of other families in the same region found a way to educational advantages. Only 20 individuals learned a trade and 10 of these did so in state's prison. It is estimated that up to 1877 this experiment in human breeding had cost the state of New York over a million and a quarter dollars, and the end is by no means yet in sight. b. The descendants of Jonathan Edwards In striking contrast to the case of Max Jukes is that of Jonathan Edwards, the eminent divine, whose famous progeny Winship describes as follows: " 1394 of his descendants were identified in 1900, of whom 295 were college graduates ; 13 presidents of our greatest colleges, besides many principals of other important educational institutions ; 60 physi- cians, many of whom were eminent; 100 and more clergymen, missionaries, or theological professors; 75 were officers in the army and navy; 60 were prominent authors and writers, by whom 135 books of merit were written and published and 18 impor- tant periodicals edited; 33 American States and several foreign countries and 92 American cities and many foreign cities have profited by the beneficent influence of their eminent activity ; 100 and more were lawyers, of whom one was our most eminent professor of law; 30 were judges; 80 held public office, of whom one was vice-president of the United States; 3 were United States senators; several were governors, Members of Congress, framers of state THE APPLICATION TO MAN 229 constitutions, mayors of cities, and ministers to for* eign courts; one was president of the Pacific Mail Steamship Company ; 15 railroads, many banks, in- surance companies, and large industrial enterprises have been indebted to their management. Almost if not every department of social progress and of public weal has felt the impulse of this healthy, long-lived family. It is not known that any one of them was ever convicted of crime." c. The Kcdlikak Family A more convincing experiment in human heredity than the foregoing, since it concerns the descendants of two mothers and the same father, is furnished by the recently published history of the " Kallikak " family. 1 During Revolutionary days, the first Martin Kalli- kak, the name is fictitious, who was descended from a long line of good English ancestry, took advantage of a feeble-minded girl. The result of their indulgence was a feeble-minded son who be- came the progenitor of 480 known descendants of whom 143 were distinctly feeble-minded, while most of the others fell below mediocrity without a single instance of exceptional ability. "After the Revolutionary war, Martin married a Quaker girl of good ancestry and settled down to live a respectable life after the traditions of his forefathers. From this legal union with a normal woman there have been 496 descendants. All of 1 " The Kallikak Family." H. H. Goddard. The Macmillan Co. 230 GENETICS these except two have been of normal mentality and these two were not feeble-minded. . . . The fact that the descendants of both the normal and the feeble-minded mother have been traced and studied in every conceivable environment, and that the re- spective strains have always been true to type, tends to confirm the belief that heredity has been the determining factor in the formation of their respec- tive characters." 4. MORAL AND MENTAL CHARACTERS BEHAVE LIKE PHYSICAL ONES These instances of human breeding show unmis- takably that "blood counts" in human inheritance, even though the hereditary unit characters that lead to these general results have not yet been analyzed with the clearness that is possible in dealing with the characters of some animals and plants. There is of course no question of moral and mental traits in plants, and the role that these play in animals is not easy to determine; but in man the case is undoubtedly much more important and complex, since mental and moral characteristics have a large share in making man what he is. There is, however, no fundamental scientific distinction which can be drawn between moral, mental, and physical traits, and they are undoubtedly all equally subject to the laws of heredity. For instance, as an illustration of the heritability THE APPLICATION TO MAN 231 of non-physical traits, in the Jukes pedigree three of the daughters of Max impressed their peculiar moral and mental characteristics in a distinctive way upon their offspring. To quote Davenport : "Thus in the same environment, the descendants of the illegitimate son of Ada are prevailingly criminal ; the progeny of Belle are sexually immoral; and the offspring of Effie are paupers. The difference in the germplasm determines the difference in the prevailing trait." 5. THE CHARACTER OF HUMAN TRAITS Of the mental, moral, and physical traits which are heritable in man, some must be regarded as generally desirable, some as indifferent, and others as defects to be avoided if possible. In general the majority of human traits, those which together make up man as distinguished from other animals, do not particularly claim the attention because they are so universal. Some which stand out from the mass, such as the physical traits of eye-color and the color and character of hair, may be regarded as indifferent so far as the welfare of the individual is concerned, while others like skin color and certain racial features that characterize particular strains of "blood" may, under certain circumstances, work a social handicap upon their possessors according to the traditions of the community in which they appear. A long list of desirable mental traits might be enumerated that seem in a general way to be subject to the laws of inheritance, although they have not 232 GENETICS yet undergone the careful analysis demanded by modern genetics which deals in unit characters rather than in lump inheritance. Musical, literary, or artistic ability, for example, mathematical aptitude and inventive genius, as well as a cheerful disposition or a strong moral sense are probably all gifts that come in the germplasm. They may each be developed by exercise or re- pressed by want of opportunity, nevertheless they are fundamentally germinal gifts. A genius must be born of potential germplasm. No amount of faithful plodding application can com- pensate for a lack of the divine hereditary spark at the start. 6. HEREDITARY DEFECTS Undesirable hereditary traits are usually defects due to the absence of some character. For instance, albinism, which occurs in several kinds of animals and also in man in one out of every 20,000 individuals (according to Elderton), is due to the absence of pig- ment in the skin, hair and eyes. Albinic individuals have poor eyesight because they are unable to stand strong light, being without protective pigment in the eyes. This peculiarity of albinism behaves as a recessive character both in man and in other animals. An albinic individual may, therefore, marry a normal individual without fear of producing albino children, although the children of such a mating would carry heterozygous germplasm with respect to albinism, THE APPLICATION TO MAN 233 and in cousin marriages might subsequently produce some albino children. Davenport, in his recent work on "Heredity in Re- lation to Eugenics," brings together a long catalogue of human hereditary defects, although in most instances they are extremely difficult of accurate analysis. This is the case, first, because these defects so often probably depend upon a combination of determiners rather than upon a single one, and, sec- ond, because the available data are usually scattered and incomplete. Deafness, for example, is a defect which is heredi- tary though exactly to what degree, it is at present impossible to state. The following table taken from the extensive work of Fay (1898) upon "Marriage of the Deaf in America" gives some idea of the results of different matings lumped together statistically. CONDITION OF PARENTS PERCENTAGE OP DEAF OFFSPRING Both born deaf 25.9 One born deaf, one with acquired deafness 6.3 One born deaf, one normal 11.9 Both with acquired deafness 2.3 One with acquired deafness, one normal . . 2.2 That two parents born deaf do not produce more than 26 per cent of deaf children is probably due to the fact, first, that each parent is in all likelihood heter- ozygous for deafness and that, second, the same com- 234 GENETICS bination of factors which is the cause of the parental defect on either side of the pedigree does not happen to recombine after segregation to form the new individ- ual. Deafness will be produced in the offspring only when matings occur in which the proper factors are combined. Such an undesirable result is much more likely to happen if both parents come from the same, or related, hereditary strains than if they are derived from families in no way connected by blood. Herein lies the biological objection to cousin marriage which tends to bring together, and thus to perpetuate, like defects. Outcrossing, on the contrary, through the law of dominance, tends to conceal defects and to prevent their expression. Many other cases of human defects, such as im- becility or insanity, are extremely difficult of analysis from the standpoint of heredity because, in the first place, the defective conditions descriptively included under these vague terms are made up of a multitude of diverse conditions each of which must have a different array of determiners and, in the second place, because any one definite sort of insanity or imbecility may be conditioned by a variety of factors. However, the difficulty of the problem is no reason for abandoning the attempt to reach its solu- tion and to learn, if possible, "whence come our 300,000 insane and feeble-minded, our 160,000 blind or deaf, the 2,000,000 that are annually cared for by our hospitals and Homes, our 80,000 prisoners and the thousands of criminals that are not in prison, THE APPLICATION TO MAN 235 and our 100,000 paupers in almshouses and out " (Davenport) . 7. THE CONTROL OF DEFECTS The method of possible control of human defects depends upon whether they are positive or negative, that is, dominant or recessive. In those cases where a given defect is due to a single determiner the Mendelian expectation for the possible offspring arising from various matings is indicated in the fol- lowing table in which D stands for the defect and d for its absence : THE MENDELIAN EXPECTATION FOR DEFECTS 1 IF THE DEFECT is POSITIVE (dominant) IF THE DEFECT is NEGATIVE (recessive) When both parents show the defect DD X DD = all DD dd X dd = all dd 2 3 DD X Dd = i DD + 1 Dd DdXDd=DD+lDd+ldd When one parent only shows the defect 4 DD X dd = all Dd dd X DD = all Dd 6 Dd X dd = J Dd + J dd dd X Dd = J Dd + J dd When neither parent shows the defect 6 dd X dd = all dd DD X DD = all DD 7 Dd X DD = J DD + } Dd 8 DdXDd = lDD+lDd+ldd If the defect is positive and in a duplex or homo- zygous condition in one parent, as in 1, 2, and 4 above, all the offspring will possess it regardless of the germinal constitution of the other parent. In 236 GENETICS two cases only, namely, in 3 and 5, where the de- fective parent is heterozygous, is there any chance of unaffected offspring, and even in these cases the defect is quite as likely to appear as not. It is ob- vious that the only way to rid germplasm of a dominant defect is by continued mating with recessive individuals. By this method it is possible in time to shake off the defect. When it once dis- appears in any individual, it will never return unless crossed back to a similar defective dominant strain. In other words, such a recessive extracted from a heterozygous ancestry will breed just as true as a recessive which was pure from the start. In both instances there is an entire absence of the character in question, and it is clear that this character can thereafter never again reappear, since something cannot be derived from nothing. On the other hand, if a defect is negative, depending upon the absence of a normal dominant determiner, as is usually the case with defects, it behaves as a Mendelian recessive, that is, it is always apparent in individuals developing from the homozygously de- fective germplasm. It is certain, for example, that an imbecile which has arisen from homozygous defective germplasm carries only the determiner for imbecility in his own germplasm, and when two such recessives mate, noth- ing but imbecile offspring can result, for recessives breed true. Nothing plus nothing equals nothing. An illustration of this principle is given in the fol- lowing pedigree (Fig. 72) furnished by Goddard, 1910. THE APPLICATION TO MAN 237 The result is quite different, however, when one parent only shows the defect. If the other parent is a normal homozygote, as in case 4 of the accompanying table, all the offspring will be normal in appearance, but with the bar sinister of defectiveness in their germplasm, while if the other parent is heterozygous (Case 5), one half of the progeny will be defective. Finally, when neither parent shows defectiveness FIG. 72. Pedigree chart illustrating the law that two defective parents have only defective offspring. A, alcoholic ; C, criminalistic ; d, died ; F, feeble-minded ; T, tubercular. After Goddard. but one carries the defect as a heterozygote (Case 7), then there will be no defective children, while if both parents are heterozygous there is one chance in four that the offspring will be defective. As a matter of fact, defectives usually mate with defectives for the simple reason that normals ordi- narily avoid them, so it comes about that streams of poor germplasm naturally flowing together tend to the inbreeding of like defects. 238 GENETICS Davenport 1 lays down the.following general eugenic rules for the guidance of those who would produce offspring wisely : "If the negative character is, as in polydactylism and night-blindness, the normal char- acter, the normals should marry normals, and they may be even cousins. If the negative character is abnormal, as imbecility and liability to respiratory dis- eases, then the marriage of two abnormals means prob- ably all children abnormal ; the marriage of two nor- mals from defective strains means about one quarter of the children abnormal ; but the marriage of a normal of the defective strain with one of a normal strain will probably lead to strong children. The worst possible marriage in this class of cases is that of cousins from the defective strain, especially if one or both have the defect. In a word, the consanguineous marriage of persons one or both of whom have the same undesirable defect, is highly unfit, and the mar- riage of even unrelated persons who both belong to strains containing the same undesirable defect is un- fit. Weakness in any characteristic must be mated with strength in that characteristic; and strength may be mated with weakness." 8. INBREEDING The whole matter of inbreeding and the part it plays in emphasizing defects has received a fresh interpretation in the light of Mendelism. There is a widespread popular belief that inbreed- ing is injurious and that it is necessary to outcross 1 Davenport. Rep. of Amer. Breeders' Assoc., Vol. VI, p. 431, 1910. THE APPLICATION TO MAN 239 in order to maintain the vigor and avoid the defects of any line. In the case of mankind, consanguineous marriage of various degrees has long been forbidden by law or custom in many races, particularly among the Jews, Mohammedans, Indians and Romans. On the other hand, the Persians, Greeks, Phoenicians and Arabs have freely practised inbreeding, while one of the longest of known human pedigrees, a royal line of Egypt, was notorious for close inbreeding, even to the mating of brother and sister. There has been a greater degree of inbreeding in the Puritan stock of New England than is commonly realized. David Starr Jordan points out that a child of to-day, supposing no inbreeding of relatives had occurred, would have had in the time of William the Conqueror, thirty generations ago, 8,598,094,592 living ancestors. If this theoretical supposition were really so, it would seem quite possible for every New Englander to-day to have had at least one an- cestral representative who won glory under William. The difference between the unthinkable number given above and the actual number of probable ancestors alive thirty generations ago emphasizes the fact that inbreeding must have occurred freely. There are, indeed, various well-known provisions in nature to insure inbreeding. The majority of plants are probably self-fertilized while hermaphro- ditic animals, which sometimes at least are self- fertilized particularly among the lower forms, are very common. Nature has secured, on the other hand, often by 240 GENETICS elaborate devices, a separation of the sexes, especially among the higher organisms, and in consequence there has arisen an unavoidable necessity of out- crossing. The intricate adaptations existing between insects and flowers, for example, seem to be directed entirely toward insuring out crossing among plants. 9. EXPERIMENTS TO TEST THE EFFECT OF IN- BREEDING Numerous experiments to test the effect of in- breeding have been carried out upon various or- ganisms. Darwin, for instance, planted morning-glories, Ipomoea, derived from the same stock of seeds, in two beds which were laid out side by side, that is, in an environment as nearly the same as possible, but with half of the beds screened from insects which usually transfer pollen from flower to flower. In the screened half where all insects were excluded the flowers were of necessity self-fertilized, while in the exposed half they were presumably cross-pol- linated by the insects which had free access to them. The seeds produced in the two beds were kept sep- arate and the experiment was continued for ten years, so that at the end of that time two lots of morning- glories, one self -fertilized for ten generations and the other presumably cross-pollinated for the same length of time, were obtained for comparison. The crite- rion Darwin used was the vigor of the plants as shown by the length of the vine. He found that the THE APPLICATION TO MAN 241 cross-pollinated plants were to the self-pollinated ones as 100 to 53, and his conclusion was conse- quently, that cross-pollination is beneficial and self- pollination is detrimental. Ritzema-Bos inbred rats for twenty generations. For the first ten generations the average number of young per litter was 7.5, while for the last ten genera- tions it fell to 3.2. Weismann inbred mice for twenty-nine genera- tions and obtained a parallel result. For the first ten generations the average number per litter was 6.1, for the second ten generations 5.6, and for the last nine generations 4.2. Shull found in growing Indian corn that loss of vigor results from continual self-fertilization, and many breeders have had similar experiences with other plants and animals. On the other hand, in the case of the pomace fly, Drosophila, Castle inbred brother and sister for fifty-nine generations without diminishing the fer- tility of the line. No arbitrary law with respect to the effects of inbreeding upon vigor and fertility can be laid down, therefore, which will apply equally to all cases. 10. THE INFLUENCE OF PROXIMITY Inbreeding is often the result of proximity. In- sular or isolated communities, slums in cities, where those of one language herd together, or hovels in the backwoods, where degenerates of a kind are kept in intimate association, as well as asylums of various R 242 GENETICS sorts in which similar defectives are promiscuously housed under the same roof, are all potent agencies to insure human inbreeding. Similarly, localities which have been devastated by migrations of the most effective blood, as, for example, parts of Ireland or many rural villages in New England, are frequently characterized by a population showing a large percentage of defective- ness. The able-bodied and ambitious go forth into the world to seek their fortunes, while the deficient in body or spirit are left behind where, under the spell of proximity, they perpetuate their deficiencies. The part that improved transportation has played in mixing up populations and in counteracting the effects of stagnation on human heredity, through in- breeding under the inertia of proximity is very great. Before the days of railroads, cousin-marriages were much more frequent than they are now. From a biological point of view there is something to be said in favor of the rape of the Sabines in the past and for the pursuit of American heiresses by European nobility to-day. 11. INBREEDING IN THE LIGHT OF MENDELISM Inbreeding in itself may not necessarily be injuri- ous. The consequence of inbreeding as shown by the working of Mendelian laws is that latent or recessive characters tend to become homozygous and so brought to the surface, while outcrossing brings about the formation of heterozygous traits which THE APPLICATION TO MAN 243 mask recessive characters and render them ineffec- tive. Cousin-marriages, although producing a high per- centage of defects, do not necessarily reproduce un- desirable traits. They simply bring out latent or recessive characters for the reason that under these conditions defect meets defect instead of the opposite normal condition which would dominate the defect and cause it not to appear. Since a recessive trait is properly regarded as the absence of a positive dominant character, it more frequently stands for an undesirable feature than otherwise. Thus it comes about that inbreeding, by combining negative features, may "produce" a defective strain. Outcrossing always increases heterozygous com- binations in the germplasm and covers up undesirable recessive traits through the introduction of addi- tional dominant traits. Inbreeding, on the contrary, tends to simplify the germplasm, that is, to make it more homozygous, and so to bring recessive defects to the surface. CHAPTER XII 1. How MANKIND MAY BE IMPROVED THERE are two fundamental ways to bring about human betterment, namely, by improving the in- dividual and by improving the race. The first method consists in making the best of whatever heritage has been received by placing the individual in the most favorable environment and developing his capacities to the utmost through education. The second method consists in seeking a better heritage with which to begin the life of the individual. The first method is immediate and urgent for the present generation. The second method is concerned with ideals for the future, and consequently does not usu- ally present so strong an appeal to the individual. The first is the method of euthenics, or the science of learning to live well. The second is eugenics, which Galton defines as "the science of being well born." These two aspects of human betterment, however, are inseparable. Any hereditary characteristic must be regarded, not as an independent entity, but as a reaction between the germplasm and its environment. The biologist who disregards the fields of educational endeavor and environmental influence, is equally at 244 HUMAN CONSERVATION 245 fault with the sociologist who fails sufficiently to real- ize the fundamental importance of the germplasm. Without euthenic opportunity the best of heri- tages would never fully come to its own. Without the eugenic foundation the best opportunity fails of accomplishment. The euthenic point of view, however, must not distract the attention now, for the present chapter is particularly concerned with the program of eugenics. 2. MORE FACTS NEEDED Since the point of attack in human heredity must be largely statistical, it is of the first importance to collect more facts. Our actual knowledge is con- fused with a mass of tradition and opinion, much of which rests upon questionable foundations. The great present need is to learn more facts ; to sift the truth from error in what is already known; and to reduce all these data to workable scientific form. Much progress is being made in this direction, owing to the impetus given by the revival of Mendel's illuminating work, but as yet the science of eugenics is in its infancy. The most systematic and effective attempt in this country to collect reliable data concerning heredity in man has been initiated by the Eugenics Section of the American Breeders' Association under the secretaryship of Dr. C. B. Davenport. In 1910 the Eugenics Record Office, with a staff of expert field and office workers and an adequate equipment of fire-proof vaults, etc., for the preservation of records, 246 GENETICS was opened at Cold Spring Harbor, Long Island, New York, with Mr. H. H. Laughlin as superintendent. "The main work of this office is investigation into the laws of inheritance of traits in human beings and their application to eugenics. It proffers its serv- ices free of charge to persons seeking advice as to the consequences of proposed marriage matings. In a word, it is devoted to the advancement of the science and practice of eugenics." The publica- tion of results from the Eugenics Record Office has already been begun. The Volta Bureau, founded about twenty-five years ago in Washington by Dr. Alexander Graham Bell, is collecting data with reference to deafness and has now systematically arranged particulars con- cerning the history of over 20,000 individuals. In England, also, the Gal ton Laboratory for Eugenics, founded in 1905, is systematically collecting facts about human pedigrees and publishing the results in a compendious " Treasury of Human Inheritance." Besides these special bureaus of investigation, innumerable facts about the inheritance of particular traits are being incidentally brought together and made available in various institutions and asylums throughout the world which are immediately con- cerned with the care of defectives of different types. It is in connection with such institutions for defec- tives that much of the most successful "field work'* of the Eugenics Section of the American Breeders' Association is being accomplished in the United States. HUMAN CONSERVATION 247 3. FURTHER APPLICATION OF WHAT WE KNOW NECESSARY Human performance always lags behind human knowledge. Many persons who are fully aware of the right procedure do not put their knowledge into practice. It follows, therefore, that any program of eugenics which does not grip the imagination of the common people in such a way as to become an effec- tive part of their very lives is bound to remain largely an academic affair for Utopians to quarrel and theo- rize over. It is not enough to collect facts and work out an analysis and interpretation of them, for, important as this preliminary step is, it must be followed by a convincing campaign of education. The lives of the unborn do not force themselves upon the average man or woman with the same insistency as the lives already begun. In the midst of the overwhelming demands of the present, the appeal of posterity for better blood is vague and remote. If every individual regarded the germ- plasm he carries as a sacred trust, then it would be the part of an awakened eugenic conscience to restrain that gennplasm when it is known to be defective or, when it is not defective, to hand it on to posterity with at least as much foresight as is exercised in breeding domestic animals and cultivated plants. The eugenic conscience is in need of development, and it is only when this becomes thoroughly aroused in the rank and file of society as well as among the 248 GENETICS leaders, that a permament and increasing better- ment of mankind can be expected. 4. THE RESTRICTION OF UNDESIRABLE GERM- PLASM A negative way to bring about better blood in the world is to follow the clarion call of Davenport and "dry up the streams that feed the torrent of de- fective and degenerate protoplasm." This may be partially accomplished, at least in America, by employing the following agencies : control of immi- gration; more discriminating marriage laws; a quickened eugenic sentiment ; sexual segregation of defectives ; and finally, drastic measures of asexuali- zation or sterilization when necessary. a. Control of Immigration The enforcement of immigration laws tends to debar from the United States not only many unde- sirable individuals, but also incidentally to keep out much potentially bad germplasm that, if admitted, might play havoc with future generations. For example, during the year of 1908, 65 idiots, 121 feeble-minded, 184 insane, 3741 paupers, 2900 individuals having contagious diseases, 53 tuber- culous individuals, 136 criminals, and 124 prostitutes were caught in the sieve at Ellis Island alone and turned back from this country by the immigration officials. These 7000 and more individuals probably were the bearers of very little germplasm that we are nationally not better off without. HUMAN CONSERVATION 249 Eugenically, the weak point in the present applica- tion of immigration laws is that criteria for exclusion are phenotypic in nature rather than genotypic, and consequently much bad germplasm comes through our gates hidden from the view of inspectors because the bearers are heterozygous, wearing a cloak of desirability over undesirable traits. It is not enough to lift the eyelid of a prospective parent of American citizens to discover whether he has some kind of an eye-disease or to count the contents of his purse to see if he can pay his own way. The official ought to know if eye-disease runs in the immigrant's family and whether he comes from a race of people which, through chronic shiftlessness or lack of initiative, have always carried light purses. In selecting horses for a stock-farm an expert horseman might rely to a considerable extent upon his judgment of horseflesh based upon inspection alone, but the wise breeder does more than take the chances of an ordinary horse trader. He wants to be assured of the pedigree of his prospective stock. It is to be hoped that the time will come when we, as a nation, will rise above the hazardous methods of the horse trader in selecting from the foreign appli- cants who knock at our portals, and that we will exercise a more fundamental discrimination than such a haphazard method affords, by demanding a knowledge of the germplasm of these candidates for citizenship, as displayed in their pedigrees. This may possibly be accomplished by having trained inspectors located abroad in the communi- 250 GENETICS ties from which our immigrants come, whose duty it shall be to look up the ancestry of prospective applicants and to stamp desirable ones with approval. The national expense of such a program of genealogi- cal inspection would be far less than the mainte- nance of introduced defectives, in fact it would greatly decrease the number of defectives in the country. At the present time this country is spending over one hundred million dollars a year on defectives alone, and each year sees this amount increased. The United States Department of Agriculture already has field agents scouring every land for desirable animals and plants to introduce into this country, as well as stringent laws to prevent the im- portation of dangerous weeds, parasites, and organ- isms of various kinds. Is the inspection and super- vision of human blood less important ? b. More Discriminating Marriage Laws Every people, including even the more primitive races, make customs or laws that tend to regulate marriage. Of these, the laws which relate to the eugenic aspect of marriage are the only ones that concern us in this connection. "Marriage," says Davenport, "can be looked at from many points of view. In novels as the climax of human courtship ; in law largely as two lines of property descent; in society, as fixing a certain status ; but in eugenics, which considers its biological aspect, marriage is an experiment in breeding." Certain of the United States have laws for- HUMAN CONSERVATION 251 bidding the marriage of epileptics, the insane, habit- ual drunkards, paupers, idiots, feeble-minded, and those afflicted with venereal diseases. It would be well if such laws were not only more uniform and widespread, but also more rigidly enforced. It is quite true that marriage laws in themselves do not necessarily control human reproduction, for illegitimacy is a factor that must always be reckoned with; nevertheless such laws do have an important influence in regulating marriage and consequent reproduction. Marriage laws may, however, sometimes bring about a deplorable result eugenically, as in the case of forced marriage of sexual offenders in order to legalize the offense and "save the woman's honor." To compel, under the guise of legality, two defective streams of germplasm to combine repeatedly and thereby result in defective offspring just because the unfortunate event happened once illegitimately, is fundamentally a mistake. Darwin says : "Except in the case of man himself hardly any one is so ignorant as to allow his worst animals to breed." c. An Educated Sentiment A far more effective means of restricting bad germplasm than placing elaborate marriage laws upon our statute-books is to educate public sentiment and to foster a popular eugenic conscience, in the absence of which the safeguards of the law must forever be largely without avail. Such a sentiment already generally exists to a 252 GENETICS large extent with respect to incest, and the marriage of persons as noticeably defective as idiots or those afflicted with insanity, and also in America with re- spect to miscegenation, but a cautious and intelligent examination of the more obscure defective traits, exhibited in the somatoplasms of the various mem- bers of families in question, is largely an ideal of the future. Under existing conditions non-eugenic con- siderations such as wealth, social position, etc., often enter into the preliminary negotiations of a marriage alliance, but an equally unromantic caution with reference to the physical, moral, and mental charac- ters that make up the biological heritage of con- tracting parties is less usual. The scientific attitude is not necessarily opposed to the romantic way of looking at things. Science is simply "organized common sense," and romance, that dispenses with this balance-wheel, although it may be entertaining and always exciting at first, is sure to be disappointing in the end. Marriages may be "made in heaven," but, as a matter of fact, children are born and have to be brought up on earth. It follows without saying that it will be much easier to stamp out bad germplasm when an educated senti- ment becomes common among all people everywhere. d. Segregation of Defectives Persons with hereditary defects, such as epileptics, idiots, and certain criminals, who become wards of the state, should be segregated so that their germ- plasm may not escape to furnish additional burdens HUMAN CONSERVATION 253 to society. "We have become so used to crime, disease and degeneracy that we take them for nec- essary evils. That they were in the world's igno- rance, is granted. That they must remain so, is denied" (Davenport). "The great horde of defectives once in the world have the right to live and enjoy as best they may whatever freedom is compatible with the lives and freedom of other members of society," says Kellicott, but society has a right to protect itself against repe- titions of hereditary blunders. There is one grave danger connected with the administration of our humane and commendable philanthropies toward the unfortunate, for it fre- quently happens that defectives are kept in insti- tutions until they are sexually mature or are partly self-supporting, when they are liberated only to add to the burden of society by reproducing their like. Furthermore, if defectives of the same sort are collected together in the same institutions, unless sexual segregation is strictly maintained, they may by the very circumstance of proximity tend to re- produce their kind just as defectives in any isolated community tend to multiply. David Starr Jordan cites the interesting case of cretinism which occurs in the valley of Aosta in northern Italy, to prove the wisdom of the sexual segregation of defectives. Cretinism is an hereditary defect connected with an abnormal development of the thyroid gland which results in a peculiar form of idiocy usually associated with goitre. 254 GENETICS "In the city of Aosta the goitrous cretin has been for centuries an object of charity. The idiot has received generous support, while the poor farmer or laborer with brains and no goitre has had the sever- est of struggles. In the competition of life a pre- mium has thus been placed on imbecility and disease. The cretin has mated with cretin, the goitre with goitre, and charity and religion have presided over the union. The result is that idiocy is multiplied and intensified. The cretin of Aosta has been de- veloped as a new species of man. In fair weather the roads about the city are lined with these awful paupers human beings with less intelligence than a goose, with less decency than the pig." Whymper, writing in 1880, further observes: "It is strange that self-interest does not lead the natives of Aosta to place their cretins under such restrictions as would prevent their illicit intercourse; and it is still more surprising to find the Catholic Church actually legalizing their marriage. There is some- thing horribly grotesque in the idea of solemnizing the union of a brace of idiots, and, since it is well known that the disease is hereditary and develops in successive generations the fact that such marriages are sanctioned is scandalous and infamous.'* Since 1890 the cretins have been sexually segregated, and in 1910 Jordan reported that they were nearly all gone. e. Drastic Measures A fifth method of restricting undesirable germ- plasm in the case of confirmed criminals, idiots, HUMAN CONSERVATION 55 imbeciles, and rapists may be mentioned, namely, the extreme treatment of either asexualization or vasectomy. The latter is a minor operation con- fined to the male which occupies only a few moments and requires at most only the application of a local anaesthetic, such as cocaine. There are no disturbing or even inconvenient after effects from this operation. It consists in removing a small section of each sperm duct and is entirely effectual in preventing subse- quent parenthood. In the female the corresponding operation, which consists in removing a portion of each Fallopian tube, is much more severe, but not impracticable or dangerous. Eleven states * already have sterilization laws pro- viding for certain cases and "could such a law be enforced in the whole United States, less than four generations would eliminate nine tenths of the crime, insanity and sickness of the present generation in our land. Asylums, prisons and hospitals would decrease, and the problems of the unemployed, the indigent old, and the hopelessly degenerate would cease to trouble civilization." 5. THE CONSERVATION OF DESIRABLE GERMPLASM Not only negatively by the restriction of undesir- able germplasm, but also positively by the conser- vation of desirable germplasm, may the eugenic ideal be approached. 1 Indiana, 1907; Washington, 1909; California, 1909; Connecticut, 1909; Nevada, 1911; Iowa, 1911; New Jersey, 1911; New York, 1912; No. Dakota, 1913; Michigan, 1913; Kansas, 1913. 256 GENETICS It is possible that if some of the philanthropic endeavor now directed toward alleviating the con- dition of the unfit should be directed to enlarging the opportunity of the Jit, greater good would result in the end. In breeding animals and plants the most notable advances have been made by isolating and developing the best, rather than by attempting to raise the standard of mediocrity through the elimina- tion of the worst. One leader is worth a score of followers in any community, and the science of genetics surely gives to educators the hint that it is wiser to cultivate the exceptional pupil who is often left to take care of himself than to expend all the energies of the in- structor in forcing the indifferent or ordinary one up to a passing standard. The campaign for human betterment in the long run must do more than avoid mistakes. It must become aggressive and take ad- vantage of those human mutations or combinations of traits which appear in the exceptionally endowed. There are various ways in which this improvement of society may be brought about. a. By Subsidizing the Fit The following unconfirmed newspaper clipping illustrates the point of what is meant by subsidizing the fit so far as certain physical characteristics are concerned. "Berlin. Dec. 11, 1911. The Empe- ror is reported to be interested in a plan proposed by Professor Otto Hauser for the propagation of a fixed German type of humanity, a type which HUMAN CONSERVATION 257 will be as fixed as the Jewish in its characteristics, if the suggestions of the professor can ever be carried out. The fixed type is to be produced as follows : Only 'typical' couples are to be allowed to mate. The man is to be not more than thirty years old, the woman not over twenty-eight, and each have a perfect health certificate. The man should be at least five feet seven inches tall ; the woman not under five feet six inches. Neither the man nor the woman should have dark hair. Its tint may range from blonde to auburn. The eyes of the pair should be pure blue without any tint of brown. The complex- ion should be fair to ruddy without any suggestion of heaviness or 'beefiness.' The nose ought to be strong and narrow, the chin square and powerful, and the skull well developed at the back. The man and the woman must be of German descent and must bear a German name and speak the language of Germany. These 'mated couples' are to get a wedding gift of $125 and an additional grant for each child born. The couples may settle in the United States if they prefer." This reported at- tempt to establish a Prussian type of "Hauser blondes" at least points the way to one sort of a positive eugenic method that might possibly be employed with respect to certain physical charac- teristics. It should be remembered, however, that the eugenic ideal is not by any means confined to phys- ical traits alone. 258 GENETICS b. By Enlarging Individual Opportunity Much good human germplasm goes to waste through ineffectiveness on account of unfavorable environment or lack of a suitable opportunity to develop. Every agency which contributes toward increasing the opportunity of the individual to attain to a better development of his latent possibilities is in harmony with a thoroughly positive eugenic prac- tice. Thus better schools, better homes, better living conditions, in short, all euthenic endeavor, directly serves the eugenic ideal by making the best out of whatever germinal equipment is present in man. c. By Preventing Germinal Waste Much good protoplasm fails to find expression in the form of offspring because one or the other of possible parents is cut off either by preventable death or by social hindrances. To avoid such ca- lamities is a part of the positive program of eugenics. 1. Preventable Death War, from the eugenic point of view, is the height of folly, since presumably the brave and the phys- ically fit march away to fight, while in general the unqualified stay at home to reproduce the next gen- eration. When a soldier dies on the battlefield or in the hospital, it is not alone a brave man who is cut off, but it is the termination of a probably desirable strain of germplasm. The Thirty Years' War in HUMAN CONSERVATION 259 Germany cost 6,000,000 lives, while Napoleon in his campaigns drained the best blood of France. David Starr Jordan has presented this matter very clearly. He points out that the "man with a hoe" among the European peasantry is not the result of centuries of oppression, as he has been pictured, but rather the dull progeny resulting from generations of the unfit who were left behind when the fit went off to war never to return. Benjamin Franklin, with characteristic wisdom, sums up the situation in the following epigram: " Wars are not paid for in war time ; the bill comes later." 2. Social Hindrances There are many conditions of modern society which act non-eugenically. For instance, the increasing demands of profes- sional life prolong the period necessary for prepara- tion, which, with the "cost of high living," tends toward late marriage. In this way much of the best germplasm is very often withheld from circulation until it is too late to be effective in providing for the succeeding generation. Certain occupations such as school-teaching and nursing by women are filled by the best blood ob- tainable, yet this blood is denied a direct part in molding posterity, since marriage is either forbidden or regarded as a serious handicap in such lines of work. Advertisements concerning " unincumbered help" and "childless apartments" tell their own deplorable tale. 260 GENETICS One of the darkest features of the dark ages from a eugenic standpoint was the enforced celibacy of the priesthood, since this resulted, as a rule, in with- drawing into monasteries and nunneries much of the best blood of the times, and this uneugenic cus- tom still obtains in many quarters to-day. 6. WHO SHALL SIT IN JUDGMENT ? In the practical application of a program of eu- genics there are many difficulties, for who is quali- fied to sit in judgment and separate the fit from the unfit? There are certain strongly marked characteristics in mankind which are plainly good or bad, but the principle of the independence of unit characters demonstrates that no person is wholly good or wholly bad. Shall we then throw away the whole bundle of sticks because it contains a few poor or crooked ones ? The list of weakling babies, for instance, who were apparently physically unfit and hardly worth raising upon first judgment, but who afterwards became powerful factors in the world's progress, is a notable one and includes the names of Calvin, Newton, Heine, Voltaire, Herbert Spencer and Robert Louis Stevenson. Or, take another example. Elizabeth Tuttle, the grandmother of Jonathan Edwards whose remarkable progeny was referred to in a preceding chapter, is described as "a woman of great beauty, of tall and commanding appearance, striking carriage, of strong HUMAN CONSERVATION 261 will, extreme intellectual vigor and mental grasp akin to rapacity," but with an extraordinary deficiency in moral sense. She was divorced from her hus- band "on the ground of adultery and other immo- ralities. . . . The evil trait was in the blood, for one of her sisters murdered her own son and a brother murdered his own sister." That Jonathan Edwards owed his remarkable qualities largely to his grand- mother rather than to his grandfather is shown by the fact that Richard Edwards, the grandfather, married again after his divorce and had five sons and one daughter, but none of their numerous progeny "rose above mediocrity, and their descendants gained no abiding reputation." As shown by subsequent events, it would have been a great eugenic mistake to have deprived the world of Elizabeth Tuttle's gennplasm, although it would have been easy to find judges to condemn her. Dr. C. V. Chapin recently said with reference to the eugenic regulation of marriage by physician's certificate: "The causes of heredity are many and very conflicting. The subject is a difficult one, and I for one would hesitate to say, in a great many cases where I have a pretty good knowledge of the family, where marriage would, or would not, be desirable." Desirability and undesirability must always be regarded as relative terms more or less indefinable. In attempting to define them, it makes a great dif- ference whether the interested party holds to a puri- tan or a cavalier standard. To show how far human judgment may err as well as how radically human 262 GENETICS opinion changes, there were in England, as recently as 1819, 233 crimes punishable by death according to law. One needs only to recall the days of the Spanish Inquisition or of the Salem witchcraft persecution to realize what fearful blunders human judgment is capable of, but it is unlikely that the world will ever see another great religious inquisition, or that in applying to man the newly found laws of heredity there will ever be undertaken an equally deplorable eugenic inquisition. It is quite apparent, finally, that although great caution and broadness of vision must be exercised in bringing about the fulfilment of the highest eugenic ideals, nevertheless in this direction lies the future path of human achievement. BIBLIOGRAPHY A few recent works of a general nature are listed below. Several of these books contain bibliographies of technical papers and other original sources of information. For the past five years The American Naturalist has been particularly devoted to papers on the problems of heredity. BATESON, W. 1909. Mendel's Laws of Heredity. Cambridge University Press. Cambridge. BAUK, E. 1911. Einflihrung in die experimentelle Vererbungs- lehre. Gebriider Borntraeger. Berlin. CASTLE, W. E. 1911. Heredity in Relation to Evolution and Animal Breeding. D. Appleton and Co. New York. COULTER, CASTLE, DAVENPORT, EAST, and TOWER. 1912. Heredity and Eugenics. University of Chicago Press. Chicago. DAVENPORT, C. B. 1911. Heredity in Relation to Eugenics. Henry Holt and Co. New York. GALTON, F. 1889. Natural Inheritance. Macmillan and Co. London. GODDARD, H. H. 1912. The Kallikak Family. The Mac- millan Co. New York. CORRENS, C. 1912. Die neuen Vererbungsgesetze. Gebriider Borntraeger. Berlin. DARBISHIRE, A. D. 1912. Breeding and the Mendelian Dis- covery. Cassell and Co., Ltd. London. EAST, E. M. 1907. The Relation of Certain Biological Prin- ciples to Plant Breeding. Bull. 158. Conn. Agric. Exp. Sta. 263 264 BIBLIOGRAPHY GODLEWSKI, E. 1909. Das Vererbungsproblem im Lichte del Entwicklungsmechanik. Engelmann. Leipzig. GOLDSCHMIDT, R. 1911. Einfiihrung in die Vererbungswissen- schaft. Engelmann. Leipzig. JOHANNSEN, W. 1909. Elemente der exakten Erblichkeitslehre. Fischer. Jena. HAECKER, V. 1912. Allgemeine Vererbungslehre. Vieweg und Sohn. Braunschweig. KELLICOTT, W. E. 1911. The Social Direction of Human Evolution. D. Appleton and Co. New York. LOCK, R. H. 1909. Recent Progress in the Study of Variation, Heredity and Evolution. Murray. London. LOTSY, J. P. 1906-1908. Vorlesungen iiber Descendenz- theorien. Fischer. Jena. MORGAN, T. H. 1907. Experimental Zoology. The Mac- millan Co. New York. MONTGOMERY, T. H. 1906. The Analysis of Racial Descent hi Animals. Henry Holt and Co. New York. PUNNETT, R. C. 1911. Mendelism. The Macmillan Co. New York. REID, ARCHDALL. 1905. The Principles of Heredity. Chap- man and Hall. London. THOMSON, J. A. 1908. Heredity. Murray. London. WATSON, J. A. S. 1912. Heredity. The Dodge Publishing Co. New York. WEISMANN, A. 1904. The Evolution Theory. London. SCHALLMAYER, W. 1910. Vererbung und Auslese im Lebens- lauf der Volker. Fischer. Jena. DE VRIES, H. 1905. Species and Varieties, their Origin by Mutation. The Open Court Publishing Co. Chicago. INDEX Abnormal fertilization, 30. Abraxas, 216. Acquired callosities, 91. Acquired characters, definition of, 78. Ageniapsis, 202. Agouti, 163, 171. Albinism, 59, 151, 232. Alcoholism, 93. Alternative inheritance, 121. Amblystoma, 90, 129. American Breeders' Assoc., 245. Amitosis, 20. Ammonites, 71. Amphimixis, 55, 81. Anaphase, 20. Ancon sheep, 68. Andalusian fowl, 175, 180. Angora hair, 129. Antirrhinum, 173. Ants, 210. Aphids, 204. Appendix, vermiform, 150. Arithmetical mean, 45. Armadillo, 202. Arrested development, 149. Artistic ability, 232. Ascaris, 11, 17. Asexualization, 255. Asexual spores, 9. Atavism, 146. Autosomes, 209. Average deviation, 45. Axolotl, 90. Azaleas, double, 63. BALLS, 129. BALTZEB, 209, 220. Banana fly (see Drosophila). Banded shell, 129. Barley, 129, 154. Barrier of Linnaeus, 62. BATESON, 2, 41, 55, 69, 124, 129, 130, 133, 149, 160, 161, 171, 173, 180, 182. Battle scars, 87. BAUB, 129, 130, 173, 180. Beans, 102, 115. Beardless barley, 129. Beech leaves, 43, 49. Beech, purple, 63. BEECHEB, 70. Bees, 210. Begonia, 7. Belemnites, 71. Belgian hare, 183, 193. BELL, 246. Beneden, van, 21, 23. Bertillon system of identification, 38. BIFFEN, 129, 224. Bimodal polygons, 48. Biological inheritance, 75. Biometry, 42. Birth-marks, 87. Blending inheritance, 121, 174, 182. Blonde type, 257. BLUMENBACH, 197. Boleyn, Anne, 59. Booted poultry, 182. BOBN, 200. BOVEBI, 11, 17, 18, 24, 25, 32, 207. Brachydactyly, 129. Branched habit, 129. BBOOKS, 75. Bryonia, 222. Bryozoa, 8. BTJBBANK, 156. Calvin, 260. Canaries, 129. Capsella, 89. Captivity, effect of, 152. Carnations, double, 63. Carriers of color-blindness, 215. 265 266 INDEX CASTLE, 4, 59, 124, 141, 163, 166, 170, 171, 183, 202-205, 210, 222, 241. Castration, 210. Cats, albino, 59. tailless, 68. Cattle, birth-rate of, 201. hairless, 68. hornless, 129. roan, 176. tailless, 68. Celandine, 63. Cell, germ, 21. polar, 23. resting, 18. theory, 14. typical, 15. wall, 16. Centrosome, 17. Cerebral hernia, 69. CHAPIN, 261. Characters, congenital, 80. moral and mental, 230. unit, 61. CHAUVIN, MARIE VON, 90. Cheerful disposition, 232. Chelidonium, 63, 71. Chromatin, 16. Chromosomes, 16. extra, 207. "x," 207. "y," 209. Chromosome theory, 29. Chrysanthemum, 50. Circumcision, 87. Cirripede, 212. Clover, four-leaved, 39. COBB, 173, 183. Cochins, 182. Cocoon, yellow, 129. Colorado potato-beetle (see Lep- tinotarsa) . Color-blindness, 214. factor, 170. Comb characters, 69. Complementary factor, 159. Compound determiners, 159. Congenital characters, 80. CONKLIN, 29, 88. Consanguineous marriage, 238, 239. Constants, 44. Continuity of germplasm, 11. Convergent variation, 150. CORRENS, 124, 129, 133, 175, 205, 222. Cotton, 129. Cousin marriage, 234, 238, 242, 243. Crabs, 212. Crested head, 69, 129. Cretinism, 253. CUENOT, 164, 170, 171, 200. Cumulative factor, 159, 187. Curly hair, 137. Currant-worm, 216. Cytoplasm, 15. Daisy, 50. Daphnids, 53, 204. DARBISHIRE, 128, 177. DARWIN, 1, 23, 39, 40, 42, 52, 55. 56, 61, 67, 73, 77, 85, 86, 114, 118, 123, 124, 151, 204, 225, 240, 251. Datura, 180. DAVENPORT, 46, 59, 68, 124, 129, 148, 152, 173, 177, 178, 179, 181, 182, 231, 233, 235, 237, 245, 248, 250, 253. Deafness, 233. Death, 7. preventable, 258. Defects, hereditary, 232. Delayed dominance, 177. Department of Agriculture, 250. Dermal spines, 151. Determiners of heredity, 28. Deviation, average, 45. standard, 46. Dihybrids, 133. Dinosaurs, 71. Diluting factor, 165, 171. Dioecious plants, 220. Disease transmission, 92. Docked tails, 88. Dogs, hairless, 68. tailless, 68. trimmed ears of, 88. Dominance, 145, 174. delayed, 177. imperfect, 175. reversed, 178. INDEX 267 Dominant, 125, 148, 158. DONCASTER, 217, 218. Double flowers, 63. DREYLINCOURT, 197. DRINKARD, 129. Drosophila, 129, 241. DUGDALE, 227. Duplex dose, 133. DURHAM, 165, 171. EAST, 192. Echidna, 150. Echinoderms, 31. Echinus, 32. Education, 3. Edwards, Jonathan, 228, 260. Richard, 261. Egg, abortive, 23. cell, 21. fertilized, 24. human, 28. mature, 23. EIMER, 40. ELDERTON, 232. Elementary species, 62, 72. Environment, 2, 88. Enzyme theory, 33. Equality of sexes, 201. Erinaceus, 150. Erithizon, 150. Eugenic regulation of marriage, 261. rules, 238. sentiment, 251. Eugenics, 244. Record Office, 245. Euthenics, 244. Evening primrose (see (Enothera). Extension factor, 166, 170. Extra chromosome, 207. Extracted recessive, 148. Extra toe, 69, 178. Eye color in man, 121, 147, 177,231. in Drosophila, 129. Factor, color, 170. complementary, 159. compound, 159. cumulative, 159, 187. diluting, 165, 171. extension, 166, 170. hypothesis, 159. inhibitory, 160. intensifying, 165, 171. pattern, 163, 171. pigment, 161. restriction, 166, 170. spotting, 171. supplementary, 159, 163. uniformity, 164, 171. False reversion, 149. FARRABEE, 129. FAY, 233. Feathers, absent, 69. branched, 69. recurved, 69. twisted, 69. Feeble-mindedness, 149. Feet of Chinese women, 87. Feral animals, 150, 152. Fertilization, 24. abnormal, 30. selective, 202. Firefly, 207. Fission, 7. Flavism, 151. Fluctuation, 57. Four-leaved clover, 39. Four-o'clock, 175, 180. Four-toed guinea-pigs, 59. Franklin, Benjamin, 259. Freaks, 58. Freckles, 81. Frogs, 200. GAGE, 83. GALEN, 198. GALTON, 42, 77, 98, 117, 120-122, 151, 164, 244. Galton Laboratory for Eugenics, 246. Gamete, 23. Garden peas, 124-128, 132, 134. GEDDES, 197. Gemmules, 8. Generation, spontaneous, 6. Genetics, 2. Genotype, 113, 148. Germ-cells, 21. Germplasm, 10, 148. GODDARD, 229, 236, 237. 268 INDEX GOLDSCHMIDT, 44. Graduated variants, 108. Greyhounds, 33, 201. Greening apple, 8. Green peas, 134. Growth, 7. GBUBEB, 16. Guinea-pig, agouti, 163. albino, 59. angora, 129. brown-eyed yellow, 166. coat character, 141. four-toed, 59. HAECKEB, 17, 129. Hair color, 137, 177. Hairlessness, 68. HALLET, 152. Hare, Belgian, 183, 193. Harelip, 149. HAUSEB, 256. Hedgehog, European, 150. Heine, 260. Helix, 178. HENKING, 207. Hereditary bridge, 27. Heredity, definition of, 4. Hereford cattle, 68. Heritage, 2. Hermaphrodites, 220, 239. Heterozygote, 131. HIPPOCRATES, 198. His, 29. HODGE, 92. Homozygote, 131. HOOKE, 15. Hornless cattle, 129. Horns in sheep, 179. in cattle, 179. Horses, birth-rate of, 201. hairless, 68. pacing, 129. trotting, 129. Human betterment, 244. birth-rate, 201. egg, 28. skin color, 196. Human stature, 98. traits, 231. HUBST, 133. Hyalodaphnia, 54. Hybridization, 120. Hymenoptera, 210. Illegitimacy, 251. Imbecility, 234, 238. Immigration, 248. Immunity to rust, 129. Imperfect dominance, 175. Inachus, 212. Inbreeding, 238. Independent unit characters, 144, Indian corn, 241. Induction, parallel, 83, 93. somatic, 83. Influence of proximity, 241. Inheritance, alternative, 121. biological, 75. blending, 121. particulate, 121, 164. sex-limited, 213. Inhibitory factor, 181. Insanity, 234. Insects and flowers, 240. Instinct, 92. Intensifying factor, 165, 171. Inventive genius, 232. Ipomcea, 240. Japanese art, 37. Jamestown weed, 180. JENNINGS, 48, 103, 110, 120. JOHANNSEN, 43, 102, 110, 114, 115, 119, 155. JOBDAN, 253, 254, 259. JUKES, 227, 231. Jungle-fowl, 148. Kallikak family, 229. KAMMERER, 90. KELLICOTT, 224, 253. KELVIN, 5. KING, 200. KLEBS, 54. KOLLIKER, VON, 21. LAMABCK, 53, 76. Lamarck's evening primrose, 64. LANG, 129, 178, 180, 185, 193. Lathyrus, 160. INDEX 269 LAUGHLIN, 24C. Leghorn poultry, 177, 182. Legless lamb, 39. Leptinotarsa, 108. Liability to respiratory diseases, 238. Linnsean species, 60. Lint, 129. Literary ability, 232. Live-for-ever, 54. LOEB, 27. Lop-eared rabbit, 183. Lychnis, 67, 222. MAcDouGAL, 67, 83. Maize, 129, 192. Mantle fibers, 19. Many-celled fruit, 129. Marriage, consanguineous, 238, 239. cousin, 234, 238, 242, 243. laws, 250. MARSCHAL, 202. Mathematical aptitude, 232. Maturation, 22. McCLUNG, 207. Mean, arithmetical, 45. Melanism, 151. MENDEL, 1, 123, 130, 132, 136, 140, 143, 157, 159, 174, 202, 245. Mendel's law, 125, 127, 144, 174, 222, 225. Mental characters, 230. Membrane, nuclear, 15. Merino sheep, 68. Metaphase, 19. Mice, albino, 59. decaudalized, 88. hairless, 68. in abnormal temperature, 89. inbred, 241. intensified color, 165. piebald, 121, 164. waltzing, 129. yellow, 203. Mid-parent, 99. MIESCHER, 33. MILTON, 6. Mirabilis, 175, 180. Mitosis, 18. Mode, 45. MOHL, VON, 15. Monochorial twins, 201. Monoecious plants, 220. Monohybrids, 131, 147. MONTGOMERY, 34, 78, 80. Moral characters, 230. MORGAN, LLOYD, 86. MORGAN, T. H., 129, 205, 208. Morning-glory, 240. Mud puppy, 91. MULLENIX, 173, 183. Musical ability, 232. Mutation, 56. degressive, 59. progressive, 59. regressive, 59. theory, 56, 72. Mutilations, 87. NAGELI, 38, 40, 123. Napoleon, 259. Nat. Assoc. of British and Irish Millers, 224. Natural selection, 118, 225. Neck bare in poultry, 69. Necturus, 91. Neo-Darwinians, 77. Neo-Lamarckians, 77. Neo-Mendelian theory of sex, 205. Nettles, 129. NEWMAN, 202. Newton, 260. Nicodemus, 85. Night-blindness, 238. NILSSON-EHLE, 185. NITOBE, 37. Nuclear membrane, 15. Nucleus, 15. Nulliplex dose, 133. Nutrition theory of sex, 198. (Enothera, 64. OESTERLEBEN, 201. Oil-gland, 69. Oocyte, 23, 211. Oogonia, 23. Ophiotrocha, 212. Origin of life, 5. of species, 1. OVID, 6. 270 INDEX Ovists, 22. Oyster-borer snail, 47. Pacing habit, 129. Palatine ridges, 149. Pangenesis, 85, 114. Papaver, 63. Parallel induction, 93. Paramecium, 48, 110. Parthenogenesis, 26, 107, 210. Partial potency, 180. Participate inheritance, 121, 164. PASTEUR, 6. Pattern factor, 163, 171. PATTERSON, 202. PEARSON, 42, 43, 49. Peas, garden, 124, 132, 134, 177. sweet, 13, 160. Petunias, double, 63. PFLUGER, 200. Phacechcerus, 91. Phaseolus, 102. Phenotype, 113, 148. Phyrrhocoris, 207. Plesiosaurs, 71. Pigeons, 148, 201. Pigment factor, 161. Pigs, birth-rate of, 201. Polar cells, 23, 211. Polydactylism, 238. Pomace fly (see Drosophila). Poppy, Shirley, 63. Population, 115. Porcupine, 150. Potency, denned, 179. failure of, 180. partial, 180. total, 179. Poultry, birth-rate of, 201. booted, 182. tailless, 68, 129, 181. and the factor hypothesis, 173. and Sudan Red III, 83. and white color, 179. Predisposition to disease, 93. Prenatal influences, 87. Presence and absence hypothesis, 132. Preventable death, 258. PRICE, 129. Primitive man, 224. Primroses, double, 63. Lamarck's evening, 64. Prophase, 18. Protophyta, 7. Protoplasm, 15. Protozoa, 7, 10. Proximity, influence of, 241, Pug jaws, 69. PDNNETT, 127, 216. Pure line, 103. Puritan stock, 239. Purple beech, 63. Quadruple hybrids, 143. Quail, 92. Rabbits, albino, 59. birth-rate of, 201. color in, 13, 169. ears of, 183, 193. gray, 171. lop, 183. Radiolaria, 17. Rats, albino, 59. inbred, 241. RAYNOR, 217, 218. Recessive, 126, 148, 157. extracted, 148. REDFIELD, 79. Red flowers, 129. Regeneration, 210. Regression, 98, 151. Regressive varieties, 72. REID, 78. Reinfection, 94. Relative variability, 47. Repair, 7. Reproduction, sexual, 9, 20. Resting cell, 18. Restriction factor, 166, 170. Reversed dominance, 178. Reversion, 146. false, 149. RIDDLE, 83. RIMPAU, 153. RITZEMA-BOS, 241. Roan color of cattle, 176. Rock-pigeon, 148. Roses, double, 63. INDEX 271 Rotifers, 204. Rumplessness, 129, 181. Rust, 129. Sacculina, 212. SADLER, 198. Salamandra, 90. SATTNDERS, 129. Scalp muscles, 149. SCHENK, 198, 199. SCHLEIDEN, 14. SCHWANN, 14. Sea-urchin, 32, 207, 209, 220. Secondary sexual characters, 210. Sedum, 54. Segregation of unit characters, 130, 145, 175. of defectives, 252. Selective fertilization, 202. Serrated leaves, 129. Sex-limited inheritance, 213. Sexual reproduction, 9, 20. Sheep, 67, 179. Sheep, Ancon, 68. Shirley poppy, 63. SHULL, 67, 129, 133, 222, 241. Siamese twins, 69. Silk-worm, 129. Simplex dose, 133. SITOWSKI, 83. Skew polygons, 50. Skin color in man, 196, 231. SMITH, 212. Smooth-margined nettle leaves, 129. Smooth peas, 129, 134. Snails, 58, 129, 178, 180. Snapdragon, 129, 173. Social hindrances, 259. Somatic induction, 83. Somatoplasm, 10, 148. Species, 62. cycle, 71. elementary, 62, 72. Linnaean, 60. origin of, 1. Speculations about sex, 197. SPENCER, 92, 260. Spermatocyte, 23, 211. Spermatogonia, 23. Spermatid, 23. Spermatozoa, 21, 23. Spermists, 22. Sphcer echinus, 32. Spines, dermal, 151. Spiny ant-eater, 150. SPILLMAN, 129. Sponges, 7, 8. Spontaneous generation, 6. Spores, 9. Sports, 39, 56. Spotting factor, 171. Spotted mice, 121, 164. Sprengel, 63. Spurs, extra in poultry, 69. Standard deviation, 46. STANDFUSS, 70. Starchy kernel, 129. Starfish rays, 44. Statoblasts, 8. Stentor, 16. Sterilization laws, 255. STEVENS, 207. Stevenson, R. L., 260. STOCKARD, 39. Stocks, double, 63. Straight hair, 138. Sub-species, 62. Sudan red III, 83. Sugar beet, 155. SUMNER, 89. Sunburn, 88. Sunflower, 129. Supplementary factors, 159, 163. Survival of the fittest, 226. Susceptibility to disease, 93. to rust, 129. Sweet pea, 13, 160. Taillessness, 68. Tails, docked, 88. Tatusia, 202. Telophase, 20. TENNENT, 31. Theory, cell, 14. chromosome, 29. enzyme, 33. mutation, 56, 72. nutrition, 198. Theromorphs, 71. Thigh bones absent, 69. 272 INDEX THOMSON, 146, 197. Thumb prints, 38. THUKY, 198. Tineola, 83. Toes, crippled, 87. extra, 69, 178. webbed, 69. Toenails, absent, 69. extra, 69. Tomato, 129. Total potency, 179. TOWER, 52, 55, 108. TOYAMA, 129. Training, 2. Traits, human, 231. Treasury of Human Inheritance, 246. Trees deformed by wind, 88. Triangle of life, 1. Trihybrids, 140. Trilobites, 71. Trimmed ears of dogs, 88. Trotting habit, 129. TSCHEKMAK, VON, 124, 129. Tuberculosis, 38. Tuttle, Elizabeth, 260. Twins, 201. Two-celled fruit, 129. TYNDALL, 6. Uniformity factor, 164, 171. Unit character, 61. Urosalpinx, 47. Variability, relative, 47. Variation, abnormal, 40. continuous, 40. convergent, 150. definite, 39. discontinuous, 41, 69. fluctuating, 43. fortuitous, 39. germinal, 40. graduated, 52. harmful, 39. hereditary, 41. indefinite, 39. indifferent, 39. integral, 52. morphological, 38. multiple, 39. non-hereditary, 41. normal, 40. orthogenetic, 39. physiological, 38. psychological, 38. quantitative, 41. qualitative, 41. single, 39. somatic, 40. useful, 39. Varieties, 62, 72. Vasectomy, 255. Vermiform appendix, 150. Vestigial structures, 149. VILMORIN, 155. Viola, 37. Vitalism, 82. Volta Bureau, 246. Voltaire, 260. VRIES, DE, 56, 59, 62, 64, 67, 71, 124, 129, 154, 155. WALTER, 48, 173, 183. Waltzing habit, 129. War, 258. Wart-hog, 91. Warts and toads, 87. Wasps, 210. Weakling babies, 260. Webbed toes, 69. Weismann, 7, 10, 55, 77, 78, 81, 84, 86-88, 94, 95, 123, 241. Wheat, 129, 152, 186, 224. WHYMPER, 254. WlEDERSHEIM, 87. WILSON, 34, 35, 207, 208. Wing absent, 69. WINSHIP, 228. WOLTEHECK, 53. Wright, Seth, 67. Wrinkled kernel, 129, 134, 177. X chromosome, 207. Y chromosome, 209. Yellow mice, 203. peas, 134. YtiNG, 200. ZEDERBATJR, 89. Zygote, 24. Printed in the United States of America. r I ""HE following pages contain advertisements of a * few of the Macmillan books on kindred subjects The Science of Human Behavior Biological and Psychological Foundations BY MAURICE PARMELEE Professor of Sociology, University of Missouri Illustrated^ ismo, $2.00 This is the first book to bring together the results of the most recent work in biology, zoology, neurology in particular, in genetic and comparative psychology, and in anthropology, showing the sig- nificance of this work for the analysis of the fundamental factors in the determination of human behavior ; namely, instinct, intelligence, feeling, and the different types of social relationships. The book contains contributions which are vital to psychologists, anthropologists, and social scientists, and will be of great value to the general reader bringing together in convenient form the results of work which is of so much significance for the study of human behavior and human nature. To those engaged in educational work it will be of great use, and will be found valuable as a col- lege and university text-book in certain courses in psychology and sociology. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York Introduction to Zoology BY ROBERT W. HEGNER, PH.D. Assistant Professor of Zoology in the University of Michigan Illustrated, Cloth, I2mo, 350 pages, $1.90 Only a few animals belonging to the more important phyla, as viewed from an evolutionary standpoint, are considered in this book. They are, however, intensively studied in an endeavor to teach the fundamental principles of zoology in a way that is not possible when a superficial examination of types from all the phyla is made Morphology is not specially emphasized, but is coordinated with physiology, ecology and behavior, and serves to illustrate by a com- parative study the probable course of evolution. The animals are not treated as inert objects for dissection, but as living organisms whose activities are of fundamental importance. No arguments are necessary to justify the " type course," developed with the problems of organic evolution in mind, and dealing with dynamic as well as static phenomena. College Zoology BY ROBERT W. HEGNER, PH.D. Assistant Professor of Zoology in the University of Michigan Illustrated, Cloth, I2mo, 733 pages, $2.60 This book is intended to serve as a text for beginning students in universities and colleges, or for students who have already taken a course in general biology and wish to gain a more comprehensive view of the animal kingdom. It differs from many of the college textbooks of zoology now on the market in several important re- spects : (i) the animals and their organs are not only described, but their functions are pointed out ; (2) the animals described are in most cases native species ; and (3) the relations of the animals to man are emphasized. Besides serving as a textbook, it is be- lieved that this book will be of interest to the general reader, since it gives a bird's-eye view of the entire animal kingdom as we know it at the present time. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York The Cell in Development and Inheritance By EDMUND B. WILSON, Ph.D. Professor of Zoology, Columbia University SECOND EDITION, REVISED AND ENLARGED Illustrated, 8vo, $3.50 Since the appearance of the first edition of this work, in 1896, the aspect of some of the most important questions with which it deals has materially changed, most notably in case of those that are focussed in the centrosome and involve the phenomena of cell-division and fertilization. This has necessitated a complete revision of the book, many sections having been entirely rewritten, while minor changes have been made on almost every page. It has therefore been considerably enlarged, and upwards of fifty new illustrations have been added. The endeavor has, however, still been made to keep the book within moderate limits, even at some cost of com' prehensiveness ; and the present edition aims no more than did the first to cover the whole vast field of cellular biology. Its limitations are, as before, especially apparent in the field of botanical cytology. Here prog- ress has been so rapid that, apart from the difficulty experienced by a zoologist in the attempt to maintain a due sense of proportion in review- ing the subject, an adequate treatment would have required a separate volume. The author has therefore, for the most part, considered the cytology of plants in an incidental way, endeavoring only to bring the more important phenomena into relation with those more fully considered in the case of animals. The steady and rapid expansion of the literature of the general subject renders increasingly difficult the historical form of treatment and the cita- tion of specific authority in matters of detail. This plan has nevertheless still been followed. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York An Outline of the Theory of Organic Evolution With a Description of some of the Phenomena 'which it explains By MAYNARD M. 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