Table 68.
| NnRr. | Nnr2. | n2Rr. | n2r2. | |
| Black with traces of red in male. |
Black. | Game. | Brahma (without red). |
|
| P. ct. | P. ct. | P. ct. | ||
| Expectation. | 50 | 25 | 25 | |
| Realization. | 51 | 27 | 22 | |
In the case where both parents are F2 or F3 it is impossible to summate results, since the gametic formulæ of the different parents are so diverse; but the same types of solid blacks, black with trace of red in the males, Game-colored males and females, and Game with red replaced by white repeatedly occur. My plan of increasing red in the Dark Brahmas met with wholly unexpectedly prompt success, but not in the way anticipated. The result was not due to selection, but to the recombination of the factors necessary to make the Game plumage coloration.
(2) Production of a buff race by selection.—The second test was directed toward the production de novo of a new buff race from a Game fowl.
As is well known, all of our red and "buff" races, like the Buff Leghorn, Rhode Island Red, and others, have been derived from the Buff Cochin that came to us from China. The fact that a buff bird has, so far as I have been able to learn, not been produced in western countries indicates the probability that it can not be so produced at will; but the attempt seemed worth while.
I began with a Black Breasted Red Game because its plumage color is that of the primitive ancestor of domesticated poultry, and on that hypothesis the ancestor of the buff races. If these buff races were produced by extending the red through selection of the reddest offspring, that should be possible now as in the past.
A start in the direction of creating a buff bird would seem to require the elimination of the black. By crossing a black and red Game with a White Leghorn I got, in 1905, 2 white pullets with red on breast and some black specks. By crossing a Game Bantam (wingless) with a White Leghorn I got white birds with red present on wing-bar of male and breast of females and also some black spots.
In 1906 I mated 2 of these white (+ red) bantam hybrid hens with a hybrid cock and obtained again red on the wing-coverts of some white hybrids, while some were without red. From one of the hens I got 4 offspring, or 20 per cent of all, with buff on hackle-lacing, breast, and wing-coverts.
In 1907 I mated a prevailingly white male of the preceding year, that had red wing-bar, hackle, and breast, with the reddest females and obtained, along with pure whites and blacks and barred birds, these colors combined with red in various degrees, but not clearly in advance of the reddest of 1906. In 1908 I mated a white male, having red as in the Game, with my reddest hybrids. Again, white and white-and-buff birds appeared, but they showed no advance, except in one instance, among 138 young. This individual (No. 7950), derived exclusively from the Black-red Game and White Leghorn on one side and on the other from the White Leghorn-Game Bantam cross, had a uniform buff down. Unfortunately the chick quickly died.
The conclusion is that after three years of selection of the reddest offspring no appreciable increase of the red was observed—except for the remarkable case of one undeveloped chick with completely buff down. This, indeed, looks like a sport, or, perhaps, it is due to unsuspected factors. The experiment will be continued.
So well-nigh universal is heredity that it is justifiable to entertain a doubt whether any character may fail of inheritance. So far as my experience goes, non-inheritable characters are such as are weak in ontogeny, so that they may readily fail of development even when conditions are propitious; or else they are so complex—so far removed from simple unit-characters—that their heritability in accordance with established canons is obscured. The first case is apparently illustrated by the rumpless cock (No. 117) and the wingless fowl; the second case by lop-comb and by right-and-left alternatives in general.
Apart from the distinct characters that fall under these two categories there are the fluctuating quantitative conditions. These depend for the most part, as already pointed out, on variations in the point at which the ontogeny of a character is stopped; and the stopping-point is, in turn, often, if not usually, determined by external conditions which favor or restrict the ontogeny. Whether or not such quantitative variations are transmitted is still doubtful. Our experiment in increasing qualities, such as redness in plumage-color, by selection of quantitative fluctuations have not been successful in the sense anticipated; neither have selections of comb, polydactylism, or syndactylism. Recently, prolonged attempts at the Maine Agricultural Experiment Station to increase egg-yield of poultry by selection have been without result. Apparently, within limits, these quantitative variations have so exclusively an ontogenetic signification that they are not reproduced so long, at least, as environmental conditions are not allowed to vary widely.
The conclusions which others have reached, and upon which de Vries has laid the greatest stress, that quantitative and qualitative characters differ fundamentally in their heritability is supported by our experiments.
The criticism has often been made of modern studies in hybridization that they are really unimportant for evolution because hybridization is uncommon in nature. Even at the beginning of the new era it could be replied that, first, we did not know how common hybridization might turn out to be in nature, and, second, that certainly in human marriage and among domesticated animals and plants, intermixing of characters played a most important part, and, finally, the laws of inheritance of characters were of such grave physiological import as to deserve study wholly apart from any question of the rôle of hybridization in evolution.
The last decade of work has made clear many things that were before uncertain. We now realize that in nature hybridization may and actually does proceed extensively. Dr. Ezra Brainerd has shown how many wild "species" of Viola have arisen by hybridization, as may be proved by extracting from them combinations of characters that are found in the species that are undoubtedly ancestral to them. In such highly variable animals as Helix nemoralis and Helix hortensis it is very probable that individuals with dissimilar characters regularly mate in nature and transmit diverse combinations of characters to their progeny. Indeed, if one examines a table of species of a genus or of varieties of a species one is struck by the paucity of distinctive characters. The way in which species, as found in nature, are made up of different combinations of the same characters is illustrated by the following example, taken almost at random. Among the earwigs is the genus Opisthocosmia, of which the 5 species known from Sumatra alone may be considered. They differ, among other qualities, chiefly in the following characters (Bormans and Kraus, 1900):
The combinations of these characters that are found are as follows:
Other species occur, in other countries, showing a different combination of characters, and there are characters not contained in this list, which is purposely reduced to a simple form; but the same principles apply generally.
The bearing upon evolution of the fact that species are varying combinations of relatively few characters is most important. Combined with the fact of hybridization it indicates that the main problem of evolution is that of the origin of specific characteristics. A character, once arisen in an individual, may become a part of any species with which that individual can hybridize. Given the successive origin of the characters A, B, C, D, E, F, in various individuals capable of intergenerating with the mass of the species, it is clear that such characters would in time become similarly combined on many individuals; and the similar individuals, taken together, would constitute a new species. The adjustment of the species would be perfected by the elimination of such combinations as were disadvantageous.
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[1] Excluding 6 doubtful because from too young embryos and not observed by myself.
[2] One is reported as having a I comb; probably the limiting condition, again.
[3] E. g., Pen 813, 935 ♀, embryo from egg of May 13.
[4] I say probably of the duplex type because the cock of pen 769 had a distally split toe on the right foot, reminding somewhat of the reduced triplex type. But as the left foot had a typical duplex thumb, and the triplex is not common in Houdans, it should probably be classed as duplex.
[5] Lewis and Embleton (1908, p. 45) present strong arguments against the theory that syndactylism is due to arrested development.
[6] Davenport, 1906, page 34, Plate V.
[7] Davenport, 1906, pages 62 to 64, fig. 46.
[8] Thus Wright (1902) says the shanks of the Silkies (in England) are "slightly feathered," and Baldanus (1896) says that (in Germany) they are feathered on the outer half.
[9] Bateson and Punnett (1908, p. 28) recognize three "kinds" of recessive whites—that of the Silkie, that of the Rose-comb bantams, and that of "white birds that have arisen in the course of our experiments." White Cochins have perhaps been one of the ancestors of Rose-comb bantams; Bateson's new white lay recessive in the White Dorking and when mated to the White Silkie throws Game-colored offspring.
[10] Wright (1902, p. 401) recognizes the variability of the blues. He advises the breeder of Andalusians that: "Black and white ones [offspring] can be weeded out at once; two or three months later birds absolutely too light, or dark and smoky, can be selected."
[11] 1906, page 49, figs. 35, 37, 37a.
[12] Goodale, 1909, has shown that in Plymouth Rocks males may be and females usually are heterozygous in barring. There is thus a clear difference between the barring of the Cochin × Tosa hybrid and that of the Plymouth Rock. The question of the heterozygous nature of the female sex, fully discussed by Goodale, will be considered by me in another place. [Note at time of correcting proof.]
[13] Does the graying of human hair represent an ontogenetically advanced condition of the melanic pigment as yellow represents the embryonic condition?
[14] Davenport, 1908, page 60.
[15] By homologous matings I mean those in which the germ-plasms of both parents are in the same condition with reference to the unit-character; i. e., both either possess it pure or lack it altogether.
[16] Davenport, 1906, page 86.
[17] Mendel's expression on this subject is translated by Bateson (1902, p. 84) as follows: "Whoever studies the coloration which results in ornamental plants from similar fertilization can hardly escape the conviction that here also the development follows a definite law which possibly finds its expression in the combination of several independent color characters. (The italics are Mendel's.)
[18] "An inherent tendency to reversion is evolved through some disturbance in the organization caused by the act of crossing." (Darwin, Animals and Plants under Domestication, Chapter XIII, section, "Summary on proximate causes leading to reversion.")
[20] Davenport, 1906, page 35.
Clear printer's errors were corrected, but original spelling was not modified or harmonized.