PART IV.. Summary and Conclusions.

SUMMARY AND CONCLUSIONS.

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 in plate 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-feathering as 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 in plate 7, figs. 1 and 1a.

10. Three types of F₂ hen-feathered castrated males are shown in plate 2, figure 3, and plate 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 in plate 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 case of 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 not in most cases tell 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, the female 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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--------, 1912. Willkürliche Umwandlung von Säugetier-Männchen in Tieren mit ausgeprägt weiblichen Geschlechtscharakteren und weiblicher Psyche. Archiv. ges. Phys., CXXXXIIII.

STEPHAN, P., 1902. De l’hermaphoditisme chez les vertébrés. Annal. de la faculté des sciences de Marseille, XII.

STOCKARD, C. R., 1911. The fate of the ovarian tissues when planted on different organs. Arch. f. Ent. Mech., XXXII.

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STURTEVANT, A. H., 1911. Another sex-limited character in fowls, Science, XXXIII.

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DESCRIPTION OF PLATES.

PLATE 1.

FIG. 1. Black-Breasted Game bantam cock. He is typically cock-feathered, but, as in all games, his hackles and tail-coverts are shorter than in the cocks of other breeds. The comb was dubbed by the breeder.

FIG. 2. Black-Breasted Game bantam hen. The great contrast in color between the cock and hen is practically the same as that in the Brown Leghorn, in most races of Tosa fowls, and in the wild type Gallus bankiva.

FIG. 3. Sebright cock, “hen-feathered.” The short hackles, the rounded feathers of the back and saddle, and the shortness of the tail-coverts are characteristic features of these males. For details of individual feathers from different regions see plates 6 and 8.

FIG. 4. A castrated Sebright male. The drawing was made about a year after the operation. This particular bird developed a lighter color than did other castrated Sebrights (see plate 3, fig. 1). The entire dorsal region has changed its color, and the feathers have also changed in shape, length, etc. Note especially the very long hackle and saddle feathers (for details see plate 6, fig. 1a) and the change in the wing-bow. The tail-coverts have also grown long.

PLATE 2.

FIG. 1. F₁ hen-feathered male out of Game by Sebright. The hen-feathering in this bird is as complete as in the Sebright.

FIG. 2. F₁ female out of Game by Sebright.

FIG. 3. Castrated male originally hen-feathered (292), nearly black in color, as shown by the individual feathers of plate 7, figure 2. After castration the bird has become red above, with black iridescent tail-coverts, and deeper yellow (or red) below.

FIG. 4. Castrated F₁ male, originally like figure 1. Note especially the change in color of the whole upper surface that has become red, like that of the jungle-fowl. The tail-coverts have grown long and are now iridescent black. The breast has changed least, but is a richer yellow. The comb and wattles and ear lobes are shrunken, as in all capons.

PLATE 3.

FIG. 1. A castrated Sebright male. The operation was performed on a juvenile bird; the drawing was made a year later. The bird is typical as to the change in color that takes place in the Sebright. He was darker red than the bird shown in plate 1, figure 4. The red was more mahogany than the picture shows. The original feathers were like those in plate 6, fig. 2 (there erroneously referred to as those of light-colored Sebright).

FIG. 2. An F₂ hen-feathered very dark male. The condition of his plumage at the time of the operation is shown in this figure. The change that took place after castration is shown in the next figure.

FIG. 3. The change that took place in the bird drawn in figure 2 is shown here. The whole upper surface has become red, except the tail-coverts, which are iridescent black. Note also the change in color on the wing-bow. For the details of the feathers see plate 9, figures 1, 1a.

FIG. 4. A castrated F₂ bird that had been hen-feathered and had changed over to cock-feathering, as shown here. The color and the details of the original hen-feathering are shown in plate 9, figures 2 and 2a.

PLATE 4.

FIG. 1. One of the original Black-Breasted Game males used in the breeding experiments. Compare with colored drawing, plate 1, figure 1.

FIG. 2. A Black-Breasted Game hen used in the breeding experiments. Compare with colored drawing, plate 1, figure 2.

FIG. 3. A Sebright male. The bird was used in the later back-crosses and not in the original experiments. He is typical of his breed.

FIG. 4. A Sebright female. One of the birds used in the original experiments.

FIG. 5. An F₁ male. This bird had just reached maturity and was younger than the one drawn in plate 2, figure 1.

FIG. 6. An F₁ hen of the same age as the last. The pattern changed a little as the bird became older.

PLATE 5.

FIG. 1. An adult Sebright male for comparison with the next figure.

FIG. 2. A castrated Sebright male. This photograph shows the same bird from which the drawing, plate 1, figure 4, was made. It is the lighter colored bird referred to in the text.

FIG. 3. One of the two F₁ castrated birds. For comparison see the colored drawing in plate 2, figure 4.

FIG. 4. A castrated Sebright. This bird is darker, and in this sense more typical than figure 2.

FIG. 5. One of the castrated Sebright males which at one time after castration was as extremely cock-feathered as figure 2, but slowly “went back” towards hen-feathering, as the figure shows especially in the hackle and saddle. The details are much better shown in the feathers photographed in plate 8, figures 1, 2, 3, 4, 1a, 2a, 3a, 4a, 1b, 2b, 3b, 4b.

FIG. 6. The same bird was opened and the regenerated pieces of the testis removed. He returned later, as shown here, to full cock-feathering.

PLATE 6.

FIGS. 1, 1A. Typical old (1) and new (1a) feathers (after castration) of the same bird. This is the “lighter” male drawn in plate 1, figure 4, and photographed in plate 5, figure 2.

FIGS. 2, 2A. Typical old (2) and new (2a) (after castration) feathers of another Sebright. This bird developed after castration darker feathers than did the last bird. Its feathers were more like those that other castrated Sebrights developed. Legend on plate 6 erroneous as far as 2 and 2a are concerned.

PLATE 7.

FIGS. 1, 1A. Typical old (1) and new (1a) (after castration) feathers of an F₁ bird. (See plate 2, figures 1 and 4.)

FIGS. 2, 2A. Typical old (2) and new (2A) (after castration) feathers of bird shown in plate 3, figures 2 and 3 (No. 292).

PLATE 8.

Typical feathers of “dark” Sebright (1, 2, 3, 4) that after incomplete castration changed to cock-feathering (1a, 2a, 3a, 4a), then later, as pieces of the testes that had been left behind in the old situs regenerated, began to go back towards hen-feathering (1b, 2b, 3b, 4b). The bird was then opened again, and the regenerated pieces removed, when it again became cock-feathered (1c, 2c, 3c, 4c), and has so remained for more than a year.

PLATE 9.

FIGS. 1, 1A. Typical feathers of hackle and saddle from hen-feathered bird (No. 68) plate 3, figure 2, that changed over to the cock-feathered bird of plate 3, figure 3.

FIGS. 2, 2A. Typical feathers of an F₁ male (2) that changed over partly as a result of degeneration of his testes, into a cock-feathered bird (2A). The change was not so great as it is after castration.

FIGS. 3, 3A. Typical feathers of Sebright male that slightly changed towards cock-feathering (old hackle feather missing).

PLATE 10.

FIGS. 1, 1A. Old (1) and new (1A) wing-coverts of normal Sebright (1) and castrated (1A).

FIGS. 2A, 2B. Upper row, to right, “Transitional” hackle feathers (2A), and a slightly later changed-over feather from wing-bow (2A), and from back (2B). Second row, to left, old (2), transitional (2a), and changed-over feather (2b), from saddle of Sebright.

FIG. 3. Three feathers (tail-covert, wing-bow, and saddle) of an F₂ hen-feathered game-like male.

FIG. 4. A series of breast feathers from an F₂ bird. At one end of the series (the left) the feather is spangled, at the other barred.

FIG. 5. A series of breast feathers from another F₂ bird. At one end of the series (the left) the feathers are penciled, at the other end they are barred.

T. H. MORGAN PLATE 1

JULIUS BIEN, N.Y.

1. Black Breasted Game Bantam male. 2. Female. 3. Sebright male. 4. Castrated Sebright male. ]

T. H. MORGAN PLATE 2

JULIUS BIEN, N.Y.

1. Hen-feathered F₁ male. 2. F₁ female. 3. Castrated F₂ male. 4. Castrated F₁ male (Fig. 1). ]

T. H. MORGAN PLATE 3

JULIUS BIEN, N.Y.

1. Castrated Sebright male. 2. F₂ Hen-feathered male. 3. Same castrated. 4. F₂ Castrated male. ]

MORGAN PLATE 4

1. Black-Breasted Game male. 2. Black-Breasted Game female. 3. Sebright male. 4. Sebright female. 5. Hybrid male. 6. Hybrid female. ]

MORGAN PLATE 5

1. Adult Sebright male. 2. Castrated Sebright male. 3. Castrated F₁ male. 4. Another castrated Sebright male. 5. Castrated Sebright male with testes regenerating. 6. Same as 5 after second removal of testes. ]

T. H. MORGAN PLATE 6

JULIUS BIEN, N.Y.

Feathers of “light” colored Sebright (1, 2) that changed to cock-feathers after castration (1ªa, 2ª).]

T. H. MORGAN PLATE 7

JULIUS BIEN, N.Y.

Feathers of F₁ hen-feathered male before (1), and after (1ª) castration.

Feathers of a darker hen-feathered male before (2), and after (2ª) castration.]

T. H. MORGAN PLATE 8

JULIUS BIEN, N.Y.

Feathers from hen-feathered male Sebright (1, 2, 3, 4) that changed to cock-feathered male (1ª, 2ª, 3ª, 4ª) after castration; and then began to go back as the testes regenerated (1ᵇ, 2ᵇ, 3ᵇ, 4ᵇ); then changed again to cock-feathering after castration (1ᶜ, 2ᶜ, 3ᶜ, 4ᶜ).]

T. H. MORGAN PLATE 9

JULIUS BIEN, N.Y.

Feathers showing complete (1) or incomplete (2 and 3) change from hen-feathering to cock-feathering (1ª, 2ª, 3ª) after castration.]

T. H. MORGAN PLATE 10

JULIUS BIEN, N.Y.

Normal. 1, 2; transitional, 1ª, 2ª, and changed-over feathers, 2ᵇ, of Sebright, 3, 4 and 5. Feathers from F₂ birds.]

FOOTNOTES:

The expectation for 1 dominant and 1 recessive factor is so nearly the same as for 1 dominant alone that for the numbers obtained no difference between the two cases could be detected.

There is one other bird, not given in the above list, that is pure Sebright except that his legs are yellow. Until I find out by further breeding of the Sebright stock whether yellow legs are present in it, this case must remain doubtful. On the basis of a two factor color-difference one Sebright (as to color) is expected in 16 birds, and one in 64 on a three factor basis. Some Sebrights had been raised along with the back cross, hence the possibility of contamination.

Provided that the blue classification was based on the adult plumage and not on down color.

If the recessive mutation occurs first in the Z chromosome of an egg of the female it will not appear in the next generation; then if it has passed into a male, half his daughters will show it. The single factor-pair involved is carried by the sex chromosomes ZZ.

One may be either sex-linked or sex-limited so far as the evidence goes.

No mention is made by Baur that a heterozygous male instead of a pure silver male was used, although the male is made heterozygous in the formulæ.

For activity and pugnacity in hummingbirds, see Tropical Nature, pp. 130, 213.

The Naturalist in La Plata, W. H. Hudson, London, 1892, pp. 269-270.

Proceedings of the Zoological Society of London, 1885, p. 431, Quelques remarques sur le dimorphisme sexuel. Jean Stolzmann.

George W. and Elizabeth G. Peckham. Observations on Sexual Selection in Spiders of the Family Attidæ. Nat. Hist. Soc. of Wisconsin, Vol. I, 1889, pp, 46, 47.

Loddigesia mirabilis has the tail about three times as long as the body. Similar modifications are found in the genera Sappho, Cynanthus, Lesbia, Stegnura, Discura, Gouldia, et al.

Among the most remarkable of this wonderful family are the nine species of coquettes (Lophornis), which have elongated feathers, with metallic tips, springing from the sides of the neck; some have also beautiful crests. (George W. and Elizabeth G. Peckham, Additional Observations on Sexual Selection in Spiders of the Family Attidæ, Nat. Hist. Soc. of Wisconsin, 1889, vol. I, pp. 141, 142.)

Tropical Nature, p. 210. The italics are ours.

A. H. Sturtevant, Experiments on Sex Recognition and the Problem of Sexual Selection in Drosophila. Journ. Animal Behavior, Sept.-Oct. 1915, vol. 5, No. 5, pp. 352, 353.

In the eland as well as in the reindeer, in which both sexes have horns that begin in the latter at least to develop before the gonads ripen, it is stated that castration does not prevent the development of the horns in the male, but whether they are as large as in the normal male is apparently not definitely stated.

Yarrell also states that after the fallow buck has reached the height of its maturity and has 6 prongs in its antler, removal of one testis causes the next antler to have but 5 prongs.

It might be supposed that this bird was really a cock which had been changed for a hen; but the following facts put this matter beyond a doubt: First, there was no other pyed pea-fowl in the country. Secondly, the hen had knobs on her toes, which were the same after her change. Thirdly, she was as small after the change as before, therefore too small for a cock. Fourthly, she was a favorite bird, and was generally fed by the lady, and used to come for her meat, which she still continued to do after the change in the feathers.

See the latter also for references to Lacertilia and Chelonia.

Carnegie Inst. Wash. Pub. No. 278, 1918.