Chapter XIII, section, "Summary on proximate causes leading to

reversion.")

E. THE LIMITS OF SELECTION.

In the last few decades the view has been widespread that characters can be built up from perhaps nothing at all by selecting in each generation the merely quantitative variation that goes farthest in the desired direction. I have made two tests of this view, using the plumage color of poultry.

(1) _Increasing the red in the Dark Brahma × Minorca cross._--The Dark Brahma[19] belongs to the group of poultry that contains a majority of characters derived from the Aseel type. Nevertheless, its plumage is closely related to that of the Jungle-fowl, from which it may be derived on the assumption that the red part of the pattern has become, for the most part, white. However, a little red remains on the middle of the upper feathers of the wing-bar. I crossed such a bird with a Black Minorca, and, as reported in my earlier work,[20] the offspring were all black, except that the males showed some red on the wing-bar. The amount of red varied in the different males, and I decided to test the possibility of much increasing the amount of the red by selection in successive generations. So I chose the reddest cock to head the pen. In this pen (No. 632) 222 chicks were produced and grew to a stage in which their adult color could be determined. Of these 222 chicks, 160, or 72 per cent, were black, without red; 24, or 10.8 per cent, were black with some red; 38, or 11.7 per cent, were typical Dark Brahmas, and 9 others, or 4.5 per cent, were modified Dark Brahmas.

[19] Plate 11.

[20] Davenport, 1906, page 35.

The following year (pen 732) I bred a cock derived from the last year's pen, a bird that resembled much the male Dark Brahma (except that it was somewhat darker), to sundry hens, hybrids between the Dark Brahma and Minorca--some of the first and some of a later hybrid generation, but all black except that some of the 1906 birds had a little buff on the breast and the primaries. The F1 (black) × F2 (Dark Brahma) gave 51 per cent black offspring, 27 per cent with a black-and-red Game pattern, and 22 per cent with the Dark Brahma pattern devoid of red. Thus the third generation suddenly gave me a red-and-black Game-colored bird (plate 12)!

My interpretation of the foregoing results is as follows: The Dark Brahma gametic formula proves to be _CIrnwx_, whereas the Black Minorca is _C(IR)Nwx_, where (_IR_) is equivalent to, and merely a further analysis of, the _J_ of the formula of the Minorca as given in earlier sections. The _I_ stands for the Jungle pattern without red and _R_ is the red element in that pattern. Obviously _N_ and _R_ are the differential factors, 4 kinds of gametes occur in F1, and in every 16 offspring these factors are combined in the following proportions: 9 _NR_, 3 _Nr_, 3 _nR_, 1 _nr_ (compare the distribution of color types in the 222 offspring of pen 632). The F2 male selected as father of the next generation (in pen 732) was an extracted Dark Brahma in coloration and probably formed only 1 kind of gamete, _nr_; but the hens were heterozygous in respect to _N_ and _R_. Consequently 4 kinds of zygotes are to be expected in F3; and expectation was realized as indicated in table 68.

TABLE 68.

+-------------+------------+------------+-----------+-----------+ | | _NnRr._ | _Nnr2._ | _n2Rr._ | _n2r2._ | | | | | | | | | Black with | Black. | Game. | Dark | | | traces of | | | Brahma | | | red in | | | (without | | | male. | | | 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.

F. NON-INHERITABLE CHARACTERS.

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.

G. THE RÔLE OF HYBRIDIZATION IN EVOLUTION.

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):

Size: _A_, large; _a_, small. Wing-scale: _B_, brown; _b_, yellow. Antennal joints: _C_, unlike in color; _c_, uniform. Forceps at base: _D_, separated; _d_, not separated. Edge of forceps: _E_, toothed; _e_, not toothed. Fourth and fifth abdominal segments: _F_, granular; _f_, not granular.

The combinations of these characters that are found are as follows:

_Opisthocosmia ornata_: _AbcDEF_. _insignis_: _ABcDEf_. _longipes_: _AbCDEf_. _tenella_: _AbCdef_. _minuscula_: _aBCDEf_.

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.

COLD SPRING HARBOR, NEW YORK,

_May 20, 1909_.

LITERATURE CITED.

BALDAMUS, A. C. E. 1896. Illustrirtes Handbuch der Federviehzucht. Erster Band: Die Hühnervogel. 3 Aufl. bearbeitet von O. Grünhaldt. Dresden, 1896. xvi+476 pp., 102 figs.

BARFURTH, D. 1908. Experimentelle Untersuchung über die Vererbung der Hyperdactylie bei Hühnern. I. Mitth. Der Einfluss der Mutter. Arch. f. Entw.-Mech. der Org., XXVI, 631-650.

BATESON, W. 1894. Material for the Study of Variation Treated with Especial Regard to Discontinuity in the Origin of Species. London, 1894, xvi+598 pp.

1902. Mendel's Principles of Heredity: A Defence. Cambridge (Engl.), 1902, xv+212 pp.

BATESON, W., and SAUNDERS, Miss E. R. 1902. Report I to the Evolution Committee of the Royal Society. London, 160 pp.

BATESON, W., and PUNNETT, R. C. 1905. Experimental Studies in the Physiology of Heredity--Poultry. Report II to the Evolution Committee of the Royal Society. pp. 99-131.

BATESON, W., E. R. SAUNDERS, and R. C. PUNNETT. 1906. Report III to the Evolution Committee of the Royal Society, London, 53 pp.

1908. Report IV to the Evolution Committee of the Royal Society, London, 60 pp.

BORMANS, A. DE, and KRAUS, H. 1900. Forficulidæ und Hemimeridæ. Das Tierreich, 11 Lief. Berlin, XV+142 pp.

CASTLE, W. E. 1906. The Origin of a Polydactylous Race of Guinea-Pigs. Carnegie Institution of Washington Publication No. 49 (Papers of the Station for Experimental Evolution at Cold Spring Harbor, N. Y., No. 5).

CASTLE, W. E., MULLENIX, H. E., and COBB, S. 1909. Studies of Inheritance in Rabbits. Carnegie Institution of Washington Publication No. 114 (Papers of the Station for Experimental Evolution No. 13).

CORRENS, C. 1900a. G. Mendel's Regel über das Verhalten der Nachkommenschaft der Rassenbastarde. Ber. d. D. Bot. Ges., XVIII, 158-167.

1900b. Ueber Levkojenbastarde. Zur Kenntnis der Grenzen der Mendel'schen Regeln. Bot. Centralblatt, LXXXIV, 97-113. [Oct. 17].

1902. Ueber Bastardirungsversuche mit Mirabilis-Sippen. Ber. d. D. Bot. Ges., XX, 594-608.

1905. Zur Kenntnis der scheinbar neuen Merkmale der Bastarde. Ber. d. D. Bot. Ges., XXIII, 70-85.

1905. Über Vererbungsgesetze. Verhandl. Ges. D. Naturf. u. Arzte. Allg. Teil., 23 pp.

CUÉNOT, L. 1903. L'hérédité de la pigmentation chez les souris (2me note). Arch. de zool. expér. et gén. (4), I. Notes et rev., pp. xxxiii-xli.

DARWIN, C. 1876. The Variation of Animals and Plants under Domestication. Second edition, revised, vols. I, II. New York.

DAVENPORT, C. B. 1906. Inheritance in Poultry. Carnegie Institution of Washington Publication No. 52 (Papers of the Station for Experimental Evolution, No. 7), v+136 pp., 17 plates.

1907. Heredity and Mendel's Law. Proc. Washington Acad. of Sciences, IX, pp. 179-187 (July 31).

1908. Determination of Dominance in Mendelian Inheritance. Proc. Amer. Philos. Soc., XLVII, 59-63 (April).

DAVENPORT, GERTRUDE C., and C. B. 1909. Heredity of Hair Color in Man. Amer. Nat., XLIII, 193-211 (April, 1909).

DE VRIES, H. 1900. Das Spaltungsgesetz der Bastarde. Ber. der D. Bot. Ges., XVIII, 83-90 (14 March).

1902. Die Mutationstheorie. Versuche und Beobachtungen über die Entstehung der Arten im Pflanzenreich, Zweiter Band. I Lieferung, pp. 1-240.

1905. Species and Varieties: Their Origin by Mutation. Ed. by D. T. MacDougal. Chicago, 1905, xviii+847 pp.

GOODALE, H. D. 1909. Sex and its Relation to the Barring Factor in Poultry. Science, XXIX, pp. 1004-1005. June 25.

HARRISON, R. G. 1901. On the Occurrence of Tails in Man, with a Description of the Case reported by Dr. Watson. Proc. 14th Ann. Sess. Assoc. Amer. Anat., p. 141-158, 5 pls.

HURST, C. C. 1905. Experiments with Poultry. In Report II to the Evolution Committee of the Royal Society (by Bateson _et al._). London, 154 pp.

KEIBEL, F., and ABRAHAM, K. 1900. Normentafeln zur Entwicklungsgeschichte der Wirbelthiere. 2 Heft. Normentafel zur Entwicklungsgeschichte des Huhnes (Gallus domesticus). Jena, 1900, 132 pp., 3 Taf.

LANG, A. 1908. Ueber die Bastarde von Helix hortensis Müller und Helix nemoralis L. Eine Untersuchung zur experimentellen Vererbungslehre. Jena, 120 pp., 4 Taf.

LARRABEE, A. P. 1906. The Optic Chiasma of Teleosts: A Study of Inheritance. Proc. Amer. Acad. Arts and Sciences, XLII, 217-231 (Oct., 1906).

LEWIS, T., and EMBLETON, D. 1908. Split-hand and Split-foot Deformities, their Types, Origin, and Transmission. Biometrika, VI, 26-58, pls. I-VII (March).

LUTZ, F. E. 1908. The Inheritance of the Manner of Clasping the Hands. Amer. Nat., XLII, 195, 196 (March).

MENDEL, G. 1866. Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschen. Vereines in Brünn. Bd. IV, pp. 47.

SCHWALBE, E. 1906. Die Morphologie der Missbildungen des Menschen und der Tiere. I. Th. Allgemeine Missbildungslehre. Jena, xvi+230 pp.

SPILLMAN, W. J. 1909. The Nature of "Unit" Characters. Amer. Nat., XLIII, 243-248 (April).

WRIGHT, L. 1902. The New Book of Poultry. London, etc., viii+600 pp.

Transcriber's notes.

Words in italics are surrounded by _underscored-.

Clear printer's errors were corrected, but original spelling was not modified or harmonized.