PART IV.SUMMARY AND CONCLUSIONS.

“In August and September the epidermal papillæ begin to be obvious, and from this time onwards until about February a continuous increase of the epidermal papillæ and pigmentation occurs. During the greater part of this time, when the thumb-pads are attaining their characteristic rough and pigmented appearance, the testes remain inactive and unchanged—a fact which has been too readily overlooked by writers on the correlation of the primary and secondary sexual characters.”

Nussbaum (1909) and later Meisenheimer (1911) found that after castration the thumb-pads disappear. Smith confirms this report in all essential respects, although in certain details concerning the papillæhe does not agree with the two former observers. His results show that castration at the breeding-season is rapidly followed by the loss of the outer papillated layer of the thumb-pads, but castration at any other season does not have “any marked effect,” the papillæ remaining for 5 months and more in the same condition as at the time of castration. The essential point here, however, is that the excessive and even special development at the breeding-season does not take place nor is again assumed (apparently), if castration has taken place at some other time of the year.

Smith and Schuster’s attempts to transplant the testes into other males or females were unsuccessful, as the testes degenerate after a time. Auto-transplantation of the testes were more successful.

Removal of the ovary had no effect on the thumbs of the female, and even the injection of testes extracts into such females did not cause them to develop pads. Nussbaum and Meisenheimer had found that transplantation of pieces of the testes, and even injection of testes extract, into castrated frogs caused an enlargement of the thumb-pads. Smith shows that this conclusion rests on uncritical evidence. At any rate, his own more carefully planned experiments extending over the year show that the results obtained by Nussbaum and by Meisenheimer may be accounted for on other grounds than the effect of the injection or implantation.

The following statement by Smith is not without interest, since it bears directly on an important question as to how internal secretions may produce their effects.

“The deduction, therefore, which has been unduly based on Nussbaum’s experiments, that the testis of the frog contains an internal secretion, which, on being circulated in the blood, calls for the development of the secondary sexual characters, either with or without the mediation of the nervous system, is without experimental foundation.... The fact that the developmental cycle of the thumb depends for its normal course on the presence of normal living testicular tissue can be equally well explained on the theory that the testicular cells enter into a chain of metabolic processes in the body which do not pursue their normal course in the absence of the testicular cells. This disturbance of the normal metabolic processes of the body, resulting in the failure of the metabolic organs of the body to give rise to their normal products in normal quantities, may have the result of inhibiting the further development of the secondary sexual characters. The development of these latter characters may depend, therefore, not directly on the action of an internal secretion or hormone derived from the gonad, but on the elaboration of other products in other organs of the body in their due proportions. These substances may be tentatively called ‘sexual formative substances,’ but we have no reason for supposing that they are entirely devoted to sexual or reproductive purposes, and that they take no part in the ordinary metabolic processes of the body.”

The arbitrary distinctions that Smith here sets up do not seem to me to contribute anything to the situation, and in fact in the end it amounts to practically the same thing whether the hormone actsdirectly on some specific part of the body or whether in doing so it acts on other parts as well. While it is more or less customary to limit the term “hormone” to substances that do produce specific effects in a particular organ, no one would, I suppose, deny that a substance was acting as a hormone if at the same time it acted on other parts of the body also, or even if its immediate action were on some part and its ultimate action on another part of the animal. Moreover, there is nothing in the evidence appealed to by Smith that supports one rather than the other contention. It is not apparent that the simpler idea of hormone action may not still apply. Failure to implant the testes in castrated male or female, and failure of injections to produce the results sought for, may mean no more than that the experimenter failed to fulfill some one of the conditions present in the normal frog at the breeding-season. Granting that the results recorded by Nussbaum and Meisenheimer are open to the serious objections, pointed out by Smith and Schuster, the facts recorded by all three writers indicate that the maximum development of the pad takes place when the testes are at their greatest development and that the pad suddenly decreases if at this time the testes are removed. It would seem to follow that since the swelling is connected with the presence of a certain condition of the testes, its enlargement is to be referred directly to the latter, and the case comes under the general category of “secondary sexual differences,” depending on the gonad.

The secondary sexual characters ofTriton cristatuscan not, as can those of the frog, be supposed to be mechanically useful in mating, but seem to be comparable in every respect with the secondary sexual ornaments of higher animals. The work of Bresca has shown that their development is under the influence of the testes. The most important secondary sexual characters of the male are the dorsal comb and the white stripes of the tail. The comb extends along the dorsal surface of the body and of the tail (with a slight dip in the pelvic region). It is fully developed during the breeding-season, when it reaches a height of 1.5 cm. In winter it is only 0.66 mm. high, or even less. The white stripes also are fully developed in the breeding-season. They extend on each side from the cloaca to the end of the tail. In the female the white stripe is sometimes faintly seen. The angles of the tail and of the cloaca thickening are black-brown or black. The belly of the male is bright orange or “Ziegel rot”; that of the female sulphur-yellow or orange, but the difference is not constant. The upper surface of the head of the male is marbled, especially during the breeding-season almost disappearing during the rest of the year. Bresca found, when the testes were removed from sexually mature males, that in the course of a year all the important secondary sexual characters disappeared, including the comb, the white tail stripes, and the marbling of the upper surface. Removal of the ovaries did not affect the characters ofthe female. The black lower corner of the tail in the male is not changed by castration.

When the skin along the middle line of the back of the female is transplanted upon the back of a normal male (in place of his own comb) the transplanted tissue develops into a comb. In other words, under the influence of the testis, the dorsal mid-line tissues of the female change into those characteristic of the male. When pieces of skin of a male with the white tail stripes are grafted on the side of the tail of another male, the stripe remains, but when grafted similarly on a female the stripe slowly disappears. The result shows that its presence depends on the testis.

A remarkably clear case of hermaphroditism in amphibians was found by V. la Vallette St. George. He found an individual ofTriton tæniatusthat was outwardly a male with well-formed dorsal comb. In the interior were two large testes in normal position and just lateral to these on each side a large ovary. Sections showed ripe sperm in the testes and typical ova in the ovary. Sperm-ducts were present, but no oviducts. The presence of the testes will, of course, account for the development of the secondary sexual characters of the male.

Other cases amongst the Anura have been recorded by Loisel and by Marshall, Spengel, and Knappe. In the early stages of the gonad in frogs there appears to be an hermaphroditic stage in which egg mother-cells and sperm mother-cells are both present, at least in those individuals that will later become males (Kusakowitsch).

The normal hermaphroditism of certain fish (Serranus) and its rare occurrence in other species (recorded by Shattuck and Seligmann) need not be recorded here.[18]

In the Crustacea the secondary sexual characters are not marked, except in a few cases. In the amphipods, Holmes has shown direct contact plays the chief rôle in mating, and in the crayfish it has been shown by Dearborn, Andrews, and Pearse that sex recognition is largely tactile. Chidester also has shown this in crayfish. Even in crabs, and especially those living on land which have well-developed eyes and good vision, secondary sexual differences are as a rule slight and the mating instincts simple. On the other hand, the enormous chela of the male of the fiddler is supposed to be a secondary sexual difference (mainly because no other use for it has been found). Pearse suggests that the waving of this claw by the male is used as a sex signal, although he is disinclined to accept Alcock’s view that it has become “conspicuous and beautiful in order to attract the female.”

The most remarkable case known of a change in the secondary sexual characters of one sex into those of the other was discovered byGiard in 1886. As a result of infection by parasitic crustacea (e. g.,Sacculina), the male crab develops the secondary sexual characters of the female. It has been generally supposed, following Giard, that this result is due to the destruction of the testes of the male by the roots of the parasite that invades the spaces between the organs of the host, and, in the case of the testis, ultimately brings about its partial or complete destruction. Not unnaturally the results here were supposed to be parallel to those of castration in vertebrates, and received in fact the name of “parasitic castration.” More recently Geoffrey Smith has studied this phenomenon in the crabInachus, infected by the parasiteSacculina, and has reached the conclusion that the change is not due to injury or to destruction of the testes, but to a change in the metabolism of the crab brought about by the parasite.

Taking Geoffrey Smith’s case ofInachus-Sacculinaas typical, the changes brought about are as follows: The parasites attach themselves to the young crabs before the external secondary sexual differences have appeared. In the females, the effect is to cause them to develop prematurely the distinctively female characters. In the male, on the other hand, the narrow abdomen of the male changes after a molt into the broad abdomen of the female, which also develops ovigerous appendages on its ventral surface like those of the female in every detail. The larger claw of the male changes into that of the female, which is different in form as well as in size. Some years ago I ventured to raise the question as to whether these effects on the male might not be interpreted as retention of the juvenile characters rather than development of the female characters in the male. This might appear more especially the case in the somewhat more juvenile shape of the anterior abdominal appendages and possibly also in the shape of the broader abdomen; but Smith has later shown that the results can not be interpreted as juvenile, for when the changed organs are examined in detail they are found to differ from the same organs in the juvenile condition, and to be identical with those of the adult female. I think, therefore, that we must accept this interpretation of Giard and of Smith as correct. But Smith goes further and believes that the effects may be carried so far that eggs develop in the old testes; in other words, that the testis changes to an ovary. It seems to me that the evidence to support this last point should be much stronger than that advanced by Smith before we can accept this interpretation, for we lack the essential control for this evidence. In only a single case were eggs found—in the testis of a male that had been infected, but from which the parasite had fallen off, and which was presumably recovering from the effects of its presence. Now, it is known that in the testes of some male animals a few eggs may occasionally be found where there is no suspicion that the animal has changed its sex. In some crustacea, in scorpions, and in insects, isolated instances of this kind have been found. Abnormal divisionof a spermatogonial cell, of such a kind that both sex chromosomes (in the case of insects at least) got into the same cell might be expected to cause such a cell to become, even in the male, an egg-cell rather than a sperm-cell. The degenerative changes of the testes in the hermit crab caused by the parasite might be imagined to favor such abnormal division with its consequences. More significant, however, is the fact that the parasite causes the absorption of the ovary when it infects a young female, so that even all its eggs disappear. In other words, the parasite is as injurious to the peculiarly female organ as it is to the testis. Why then, one can not but ask, should an influence that causes such effects on the ovary first change a male into a female so long as it is present and then when the parasite has disappeared leave an influence behind of a kind that causes the ovary to develop—an organ which the parasite destroys when the parasite is present? Is it not more probable that only the secondary sexual organs were changed, without change in sex, the single case of eggs observed being caused in another way? This point can only be settled by direct experimentation either by removal of the testis, by injuring it, or by injection, grafting, or feeding experiments. The extent of the testis and its position make it impossible to remove it by an operation, as I have found after repeated attempts. It seemed easier to destroy it by radium. This I have tried to do, using very powerful tubes, treating the crab (fiddler crabs) for several hours. The crabs had had one claw removed—the enormously large one—and were kept until the next molt, that occurred from a week to six weeks later. In none of the cases was any change produced. The large claw of the male regenerated, of course, not full size after only one molt, but after several nearly full size and always with the peculiarities of the male crab. The abdomen and the appendages were not changed. Whether the significant cells of the testes, if there are such cells apart from the germ-cells, were destroyed, can not be told, for as yet the histological examination of the material has not been made. Until a successful operation has been done, I think we must hesitate to accept Smith’s argument, although based as it is on a series of interesting observations. His speculation is as follows:

“The reason whySacculinacauses the assumption of the adult female state inInachusis found in the facts: (1) that the roots ofSacculinaelaborate a yolk-substance from the blood ofInachusof a similar nature to that which is elaborated in the ovaries of an adultInachus; (2) that in order to elaborate this yolk-substance the roots take up from the blood ofInachusthe female sexual formation substance, which is the necessary material for forming the yolk; (3) that the female sexual formative substance being absorbed by theSacculinaroots is regenerated in excess; (4) that the presence of the female formative substance continually circulating in large quantities in the body-fluids of the infected crabs causes the production of adult female secondary sexual characters, and, when the parasite dies, of yolk-containing eggs.”

In brief, the evidence consists in showing that in the parasite a yolk-substance appears, which Smith says comes from the blood of the crabthat produces it under the influence of the parasite. Incidentally, as it were, this is said to be the same yolk-substance (but no sufficient evidence that it is the same is given) that the egg stores up inside itself, and it isassumedthat it is a formative substance that causes the cell that gets it (or contains it or secretes it—details are wanting) to become an egg-cell. It is the excess of this substance produced by the male crab, while still a male, under the influence of the parasite, that affects the abdomen and its appendages in such a way that they assume the female condition. There are too many assumptions in the argument, some of which are scarcely of a kind that our knowledge of development, incomplete as it is, can allow us to accept without more direct evidence in their support, to make this view very plausible. Until better evidence is forthcoming, I fail to be convinced by Smith’s interpretation of his facts.

Into Smith’s and Robson’s interesting observations on the blood of crabs, described in Smith’s later paper (part 7, 1911), it is not necessary to enter here, since the evidence taken as a whole offers little further in support of his view than had been already assumed. The argument on page 263 should not, however, pass unchallenged. Smith says:

“It is clear that the old and familiar idea of an internal secretion produced by the gonad being the stimulus for the development of the secondary sexual character could not be applied here, since at the time that the alterations in the secondary sexual characters take place no ovary is present to give rise to the required stimulus. It is suggested, therefore, that in some way the stimulus must reside in the roots of theSacculina,” etc.

The argument seems to imply that, since the secondary sexual characters of the female can not be produced by an ovary in the infected male, therefore theSacculinamust take the place of the ovary. But why make such a supposition, for if the testes simply keep down the development of the female characters, as Giard supposes, there is no need either for an ovary or for aSacculinato develop them. One might as well argue that since the cock does not develop the secondary sexual characters of the hen that an ovary is essential for their development—which is true, but not in the sense implied.

Stamati (1888) states that he attempted to remove the testes of adult crayfish and apparently succeeded, but since no effects are expected until after a molt occurs (that may not take place for two years or more), no results were obtained. Injections of the gonads with an acid failed, since the animals died.

In 1899 Oudemans succeeded in finding a method of removing the testes and ovaries from caterpillars, using a dimorphic species,Ocneria dispar, the gipsy moth. The results were negative; none of the secondary sexual characters of the male or female moths or the accessory organs of copulation were in the least affected by the operation. The castrated male copulated as readily with the female as did the normalmale, while the spayed females also behaved as normal individuals of that sex behave. Kellogg, in 1904, repeated the same operation in the silkworm moth on a small scale with the same results. Kopec and Meisenheimer, in 1909, repeated in a more detailed way Oudemans’s work. A further important addition was made by Kopec and by Meisenheimer. They transplanted ovaries into a castrated male and testes into a spayed female. Neither gonad produced any effect on the characters of the other sex. It is interesting to note that the testes underwent their normal development in the body of a spayed female, and even in one with the ovaries present, and that the ovary also underwent normal development in the body of the male. In other words, there is no intolerance of the tissue of one sex to the gonad of the other. This result is all the more unexpected, because other observations have shown that the color of the blood, and its chemical properties, is quite different in the male and female moths of certain species.

In the case of moths, therefore, if these cases be regarded as typical, the situation from the point of view of sexual selection is much simpler than in birds in the sense that the secondary sexual characters are directly the product of the genetic constituents of all the cells, and not influenced indirectly by the secretions from the testes or the ovaries. Sexual selection, therefore, if it is an agent in the evolution of the differences between males and females, has acted on the genetic complex to produce these effects on either sex without the result being involved in the condition of the ovary or the testes.

Regen castrated crickets,Gryllus campestris, in the larval stages and found no effects on the adult structures. The castrated males chirped like normal males and mated with the females. Spayed females were like normal females; they bored holes in the ground, but laid no eggs in them, of course, as the ovary had been completely removed.

The only genetic evidence in the group of insects, outside of the vinegar fly, relating to the secondary sexual inheritance of the secondary sexual characters is the following important experiments made by Foot and Strobell:

The male of one of the bugs,Euchistus variolarius, has a black spot on the end of the abdomen—a spot that is not present in the female. Foot and Strobell crossed a female of this species to another bug,E. servus, that lacks the spot in both sexes. The daughters had no spot, the sons a faint spot less developed than invariolarius. These inbred gave (in F₂) 249 females without a spot, 107 males with a spot, and 84 males without a spot. The results are explicable on the view that a single dominant Mendelian factor, not-sex-linked, causes the spot in the males, but the presence of the gene in the female produces no effect. The effect, therefore, is sex-limited,i. e., its expression is determined by the rest of the complex male or female.

The very important breeding experiments carried out by Goldschmidt on varieties of the gipsy moth should be referred to in thisconnection, but as I have recently reviewed these results in the paper on gynandromorphs written in collaboration with C. B. Bridges,[19]I need only refer to that account here.

[Note added April 21, 1919.]Shortly after the preceding paper was finished a theses by A. Pézard on the secondary sexual characters of birds reached me. In it the author gives an account of a number of experiments that he has made with poultry and with pheasants. His description of the changes that take place after castration are more exact and more detailed than any other so far recorded; but in general the results obtained by Pézard, through castration, are the same as those that had been obtained by others. Castration of 4 male silver pheasants are reported. No change in the plumage results, although the changes that take place in the comb and wattles are the same in kind as those observed in fowls. The sexual instincts and peculiarities of the voice and their belligerency are also lost. Similarly 4 golden pheasants that were operated on gave the same results.Three pheasants with mixed plumage (Phasianus colchicus) were examined. Their testes proved, on histological examination, to be imperfectly developed. It is not evident what relation existed between the facts and the mixed plumage. The suggestions made by Pézard seem inadequate to cover the cases.Testicular tissue transplanted into castrated cocks whose comb, wattles, etc., had undergone retrogressive changes brought about a return to the normal conditions after an interval during which the implanted nodules had begun to regenerate.Testicular extract from the cryptorchid testes of swine was injected into castrated cocks. In one case this resulted in a rapid growth in size of the comb, which, after 2 months, had reached its full size. Cessation of the injections led immediately to a cessation of growth. Before injection the bird exhibited the pacifistic characteristics of the capon, but the injections brought out little by little the aggressive behavior of the normal male. The voice reappeared and “nous assistons á une véritable crise de puberte.”A histological study of the testes of the fowl and of pheasants showed that much connective tissue is characteristic of young birds. In the adult cock, and during the mating season of the pheasant, the connective tissue becomes largely crowded out by the enlargement of the tubules. Pézard concludes that the “interstitial” cells in birds have nothing to do with the secondary sexual characters, but that these come rather under the influence of the germinal cycle of cells of the testes. The submergence of the connective-tissue cells of pheasants during the breeding-season and their reappearance during the rest of the year might appear to have some relation to the facts that I have recently described in Sebrights, but as the nuptial plumage of the male remains the same throughout the year we can not ascribe any direct influence to this tissue. Nevertheless, the different tissues of the testes in birds that show seasonal dimorphism of plumage should be carefully examined.Pézard made a few observations on hens whose ovary had been removed. His results are in accord with those of Goodale, except that he thinks that the ovary has no influence on the erectile organs (comb, etc.) which acquire in the spayed bird the samelengthas that of the normal female.Two hens showing male characteristics and a pheasant similarly affected are described. In all three cases an examination of the ovary was found to be undeveloped or abnormal.

[Note added April 21, 1919.]

Shortly after the preceding paper was finished a theses by A. Pézard on the secondary sexual characters of birds reached me. In it the author gives an account of a number of experiments that he has made with poultry and with pheasants. His description of the changes that take place after castration are more exact and more detailed than any other so far recorded; but in general the results obtained by Pézard, through castration, are the same as those that had been obtained by others. Castration of 4 male silver pheasants are reported. No change in the plumage results, although the changes that take place in the comb and wattles are the same in kind as those observed in fowls. The sexual instincts and peculiarities of the voice and their belligerency are also lost. Similarly 4 golden pheasants that were operated on gave the same results.

Three pheasants with mixed plumage (Phasianus colchicus) were examined. Their testes proved, on histological examination, to be imperfectly developed. It is not evident what relation existed between the facts and the mixed plumage. The suggestions made by Pézard seem inadequate to cover the cases.

Testicular tissue transplanted into castrated cocks whose comb, wattles, etc., had undergone retrogressive changes brought about a return to the normal conditions after an interval during which the implanted nodules had begun to regenerate.

Testicular extract from the cryptorchid testes of swine was injected into castrated cocks. In one case this resulted in a rapid growth in size of the comb, which, after 2 months, had reached its full size. Cessation of the injections led immediately to a cessation of growth. Before injection the bird exhibited the pacifistic characteristics of the capon, but the injections brought out little by little the aggressive behavior of the normal male. The voice reappeared and “nous assistons á une véritable crise de puberte.”

A histological study of the testes of the fowl and of pheasants showed that much connective tissue is characteristic of young birds. In the adult cock, and during the mating season of the pheasant, the connective tissue becomes largely crowded out by the enlargement of the tubules. Pézard concludes that the “interstitial” cells in birds have nothing to do with the secondary sexual characters, but that these come rather under the influence of the germinal cycle of cells of the testes. The submergence of the connective-tissue cells of pheasants during the breeding-season and their reappearance during the rest of the year might appear to have some relation to the facts that I have recently described in Sebrights, but as the nuptial plumage of the male remains the same throughout the year we can not ascribe any direct influence to this tissue. Nevertheless, the different tissues of the testes in birds that show seasonal dimorphism of plumage should be carefully examined.

Pézard made a few observations on hens whose ovary had been removed. His results are in accord with those of Goodale, except that he thinks that the ovary has no influence on the erectile organs (comb, etc.) which acquire in the spayed bird the samelengthas that of the normal female.

Two hens showing male characteristics and a pheasant similarly affected are described. In all three cases an examination of the ovary was found to be undeveloped or abnormal.

1. The two principal results obtained were: (a) that castration of hen-feathered Sebright males causes them to develop the full plumage characteristic of the cock-bird; (b) that complete hen-feathering is due to two dominant Mendelian genes.

2. A striking change takes place when the Sebright male is castrated (plate 1, figs. 3, 4;plate 3, fig. 1). The new feathers on the upper surface of the head, neck, back, wings, rump, and tail-coverts assume a different color and distribution of their pigment; they take on a new shape, and in those regions where in the cock the barbules are absent from a part of the margin of the feather, the same absence occurs in the castrated birds. Such feathers are present on the neck, back, wing-bow, and rump. The transition is shown in the figures inplate 6, where for comparison one of the old and one of the new feathers lie side by side. The tail-coverts in the hen-feathered bird are short, and like those in the hen do not cover the true tail. After castration they become excessively long—longer, in fact, than in many cocks—and cover the true tail feathers. The tail feathers themselves, moreover, become increased in length, as do the posterior row of feathers of the wing-coverts. On the breast and sides the change is less marked. The castrated Sebright loses his erect carriage, but how far this is due to the changes in his plumage and how far is real (as a result of a new balance due possibly to the lengthening tail and its coverts) I can not decide.

3. While castration causes the hen-feathered male to make additions in color, length, and size of many feathers, it causes at the same time the other retrogressive changes characteristic of the capon (a castrated cock-feathered bird); the comb and wattles shrink and become pale, the birds almost cease crowing, and become timid. They do not make much effort to mate with the hens, but when they do they show the usual copulatory reactions.

4. If feathers are removed at the time of castration, the new feathers show the full effect of the removal of the testes, although they must have begun to develop immediately afterward. It is suggested that by means of this delicate test the time relations of the internal secretion can be profitably studied.

5. Feathers that may have started their development at the time of the operation show the old influence at the tip of the feathers (plate 10) and the new one in the rest of the feather. The change is abrupt, although the transition is perfect.

6. Incomplete castration of the hen-feathered male leads to smaller changes in the same direction than those following complete castration.

Where such small pieces of the testis were left that complete cock-feathering followed, the bird slowly changed back to hen-featheringas the testes began to regenerate. When the regenerated pieces were removed the bird became cock-feathered again.

7. One Sebright male whose testes appear to have been completely removed did not change the character of the plumage. No testes were found on autopsy. It is suggested that some other endocrine organs have taken over the function of the testes, but as yet none such can be indicated.

8. In one case an old hen-feathered (F₁) male began to change over to cock-feathering. It was found that his testes had dwindled (probably through disease) to very small size (10 by 5 mm.).

9. The F₁ male of the cross between the Sebright and game is also hen-feathered (plate 2, fig. 1). After castration he becomes cock-feathered (plate 2, fig. 4) and shows thereby the genetic type of the heterozygous cock-feathered class in which his hen belongs. The change in this male is even more striking than that in the Sebright. The change in the individual feathers is shown inplate 7, figs. 1 and 1a.

10. Three types of F₂ hen-feathered castrated males are shown in plate 2, figure 3, andplate 3, figure 3 and figure 4. The first was a dark bird that changed to a lighter red above. The third a gray bird that became bright red; the second was a light yellow that became deep yellow, etc. The class of hens to which such males belong, as cock-feathered birds, can thus be found out by castration. In this way the F₂, and back-cross, hen-feathered cocks can be classified with the corresponding F₂ cock-feathered males.

11. In the F₂ generation, made up of birds from the direct and reciprocal crosses taken together, there were 29 hen-feathered and 26 cock-feathered males. In the back-cross (F₁ hen by game male) the classes were 2 and 7. The results seem in better accord with the assumption that two factors are present in the Sebright that stand for hen-feathering; that either alone will give hen-feathered birds (intermediate type?), but that both together give the extreme type of hen-feathering seen in the Sebright.

12. The difference in color in the two races (Sebright and Black Breasted Game bantams) is very great. The former have almost uniformly laced feathers, while the latter has the varied plumage of the jungle-fowl. The game is strongly dimorphic in color and color-pattern; the Sebright has the same type of coloration and pattern both in the male and female, but this is deceptive, as castration shows, because the castrated male is as strikingly different from the normal Sebright female as is the cock of other birds from the hen. The resemblance of male and female in this race is due to the suppression of the true male plumage by something produced in the testes. Therefore the heredity of dimorphism resolves itself here into the problem of the heredity of hen-feathering. That the female Sebright has the same genetic factors as the male is shown by the fact that she transmits hen-feathering in the same way as does the male, and also by the fact, as Darwin pointed out, that an old female Sebright whose ovaries had degenerated developed not the hen-feathered plumage of her own cock, but cock-feathered plumage like that of most male poultry.

13. The color of the F₁ birds is shown inplate 2, figs. 1 and 2. In general, the feathers are stippled, black and light yellow being the two most conspicuous ingredients. Since hen-feathering dominates, the dimorphism is absent, or at least is so slight as to not attract attention—little more, in fact, than in the Sebright race. The carriage of the male is like that of the Sebright male. The F₁ male and female are alike in the direct cross and the reciprocal, or at least no conspicuous difference is found between the two classes of hens, indicating that no important sex-linked factors are involved in the cross.

14. The F₂ birds show a great variety of color and pattern, but those obtained can be approximately grouped into 16 classes. The classes are, however, admittedly not uniform, indicating minor factors not here reckoned with. The classification of the hens is easiest; the F₂ hen-feathered males can then in many cases be referred to the proper classes; the F₂ cock-feathered males can not be accurately classified with their corresponding hens, except in the case of those that resemble the two P₁ males, the F₁ male, and those that castration experiments of the hen-feathered males have shown to belong to certain hen types.

15. Despite the admitted difficulties of classification, it is suggested that three factor-pairs of differences will cover the main color classes seen in the F₂ and in the back-cross. One or two of these seem to be incompletely dominant, since the F₁ birds are not like either parent in any single character, nor are they like the wild type in so far as this is represented by the game.

16. A histological examination of the testis of the male Sebright by Boring and Morgan has shown that it contains cells like those present in the ovary of all breeds of poultry. These cells are called luteal cells by Pearl and Boring, from their resemblance to the cells of that name found in the corpora lutea of mammals. In the mammals similar cells are supposed to produce internal secretions that act as hormones. Their function in the female bird is unknown, but the fact that after the removal of the ovary the female develops the secondary sexual plumage of the male suggests that some secretion from these cells performs this function. Their occurrence in the male Sebright and their complete absence, or paucity, in the males of other races supports strongly the view that these cells are concerned with the suppression of the secondary sexual plumage.

17. While in mammals the interstitial cells have been supposed to produce an internal secretion that causes the development of some of the secondary sexual characters of the male, and the fuller elaboration of others, in birds no such connection exists, if we except the caseof the Sebright. Castration of ordinary males does not affect deleteriously the secondary sexual plumage (although it does the comb, behavior, etc.), in fact may even enhance their effects. But, while in the mammal a secretion is necessary for the full development of the secondary sexual characters, in the Sebright a secretion inhibits certain of them. What element in the ordinary bird and in the Sebright causes the full development of the comb, wattles, sexual behavior, etc., is not known. Possibly it is the sexual elements themselves, but possibly it is a secondary influence of the luteal cells producing a contrary effect on these parts from its effects on the feathers; but possibly more than one kind of secretory cell is present in the testis of the cock.

18. The causes of the development of the secondary sexual characters are seen to be of such diverse physiological kinds that one may well hesitate to apply the same explanation as to their evolution. In fact, it is pointed out that several of the theories that have been suggested run counter to the conditions that bring about the development of the secondary sexual characters.

19. An attempt is made to give a critical review of Darwin’s theory of sexual selection in the light of the modern genetic and operative results on the secondary sexual characters of the vertebrates. It is pointed out that far from extending the general theory in its applications, the modern work has shown in the first place that the underlying conditions that call forth the development of the secondary sexual differences are so diverse in the different groups of animals that it is a priori very unlikely that this evolution can have been directed by the same external agent, such as the choice of the female, for such an assumption carries with it in several cases other implications concerning the causes of the suppression of these same characters in the female herself, etc. In the second place, it is pointed out that the problem of the excessive development of certain characters in the male whose genes are present in both sexes no longer oppresses us as it did Darwin, for it has been shown both by the genetic and by the operative work that a single factorial difference may be at the root of exceedingly great differences in the individual. Such results, while they admittedly do notin most casestell us that the differences involved have arisen at a single progressive step, show us nevertheless that such differences may depend on very simple initial differences, and if so, the entire problem becomes enormously simplified. To Darwin the excessive development of color and ornamentation appeared due to a long, slow process of evolution laboriously brought about by the female through selection of those males a little more ornamented than their fellows. To-day we have found out that in many cases the genetic composition of a male with such ornamentation and of a female without it may be almost identical, except that the genes in one chromosome are duplex in one sex and simplex in the other. Owing to this initial difference, thefemale in birds produces an internal secretion that suppresses in her the ornamentation shown by the male, and in the mammal an internal secretion produced by the testes causes the full development in the male of the secondary sexual characters. If, as seems probable, these secretions are some particular kind of substance, the condition that led to their appearance historically need not have been very complex; and if not, the problem appears simplified. It still remains to give some reasonable explanation as to why such substances should continue to be produced if their products—the secondary sexual characters—possess no “beauty” for the female. Here more work is necessary, but the modern genetic point of view may possibly give an important clue. We are coming to realize more fully that the hereditary genes generally have more than a single effect on the characters of the animal. The secondary sexual characters may, then, be only by-products of genes whose important function lies in some other direction. If, for example, the secretion produced by the cells of the male have an important influence on his output of energy, or strength, or activity, their secondary influence over certain parts of the body would not call for any further explanation on the modern view of natural selection. If the secretions of the ovary of the female bird have some direct relation to her physiological processes that are important in the development of the oviduct, for instance, it would be a matter of no importance from an evolutionary point of view if that same secretion suppresses in her the development of the high color shown by the male.

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