[Footnote 51: Darwin's _Animals and Plants under Domestication_, vol. ii. pp. 163-170.]
[Footnote 52: For a full account of these interesting facts and of the various problems to which they give rise, the reader must consult Darwin's volume on _The Different Forms of Flowers in Plants of the same Species_, chaps, i.-iv.]
[Footnote 53: See _Nature_, vol. xxi. p. 207.]
[Footnote 54: Low's _Domesticated Animals of Great Britain_, Introduction, p. lxiv.]
[Footnote 55: Low's _Domesticated Animals_, p. 28.]
[Footnote 56: _Amaryllidaceae_, by the Hon. and Rev. William Herbert, p. 379.]
[Footnote 57: _Origin of Species_, p. 239.]
[Footnote 58: _Origin of Species_, sixth edition, p. 9.]
[Footnote 59: In the _Medico-Chirurgical Transactions_, vol. liii. (1870), Dr. Ogle has adduced some curious physiological facts bearing on the presence or absence of white colours in the higher animals. He states that a dark pigment in the olfactory region of the nostrils is essential to perfect smell, and that this pigment is rarely deficient except when the whole animal is pure white, and the creature is then almost without smell or taste. He observes that there is no proof that, in any of the cases given above, the black animals actually eat the poisonous root or plant; and that the facts are readily understood if the senses of smell and taste are dependent on a pigment which is absent in the white animals, who therefore eat what those gifted with normal senses avoid. This explanation however hardly seems to cover the facts. We cannot suppose that almost all the sheep in the world (which are mostly white) are without smell or taste. The cutaneous disease on the white patches of hair on horses, the special liability of white terriers to distemper, of white chickens to the gapes, and of silkworms which produce yellow silk to the fungus, are not explained by it. The analogous facts in plants also indicate a real constitutional relation with colour, not an affection of the sense of smell and taste only.]
[Footnote 60: For all these facts, see _Animals and Plants under Domestication_, vol. ii. pp. 335-338.]
[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp. 102, 103.]
[Footnote 62: As this argument is a rather difficult one to follow, while its theoretical importance is very great, I add here the following briefer exposition of it, in a series of propositions; being, with a few verbal alterations, a copy of what I wrote on the subject about twenty years back. Some readers may find this easier to follow than the fuller discussion in the text:--
_Can Sterility of Hybrids have been Produced by Natural Selection?_
1. Let there be a species which has varied into _two forms_ each adapted to certain existing conditions better than the parent form, which they soon supplant.
2. If these _two forms_, which are supposed to coexist in the same district, do not intercross, natural selection will accumulate all favourable variations till they become well suited to their conditions of life, and form two slightly differing species.
3. But if these _two forms_ freely intercross with each other, and produce hybrids, which are also quite fertile _inter se_, then the formation of the two distinct races or species will be retarded, or perhaps entirely prevented; for the offspring of the crossed unions will be _more vigorous_ owing to the cross, although _less adapted_ to their conditions of life than either of the pure breeds.
4. Now, let a partial sterility of the hybrids of some considerable proportion of these two forms arise; and, as this would probably be due to some special conditions of life, we may fairly suppose it to arise in some definite portion of the area occupied by the two forms.
5. The result will be that, in that area, the hybrids (although continually produced by first crosses almost as freely as before) will not themselves increase so rapidly as the two pure forms; and as the two pure forms are, by the terms of the problem, better suited to their several conditions of life than the hybrids, they will inevitably increase more rapidly, and will continually tend to supplant the hybrids altogether at every recurrent severe struggle for existence.
6. We may fairly suppose, also, that as soon as any sterility appears some disinclination to _cross unions_ will appear, and this will further tend to the diminution of the production of hybrids.
7. In the other part of the area, however, where hybridism occurs with perfect freedom, hybrids of various degrees may increase till they equal or even exceed in number the pure species--that is, the incipient species will be liable to be swamped by intercrossing.
8. The first result, then, of a partial sterility of crosses appearing in one part of the area occupied by the two forms, will be--that the great majority of the individuals will there consist of the two pure forms only, while in the remaining part these will be in a minority,--which is the same as saying that the new _physiological variety_ of the two forms will be better suited to the conditions of existence than the remaining portion which has not varied physiologically.
9. But when the struggle for existence becomes severe, that variety which is best adapted to the conditions of existence always supplants that which is imperfectly adapted; therefore, _by natural selection_ the _varieties_ which are _sterile_ when crossed will become established as the only ones.
10. Now let variations in the _amount of sterility_ and in the _disinclination to crossed unions_ continue to occur--also in certain parts of the area: exactly the same result must recur, and the progeny of this new physiological variety will in time occupy the whole area.
11. There is yet another consideration that would facilitate the process. It seems probable that the _sterility variations_ would, to some extent, concur with, and perhaps depend upon, the _specific variations_; so that, just in proportion as the _two forms_ diverged and became better adapted to the conditions of existence, they would become more sterile when intercrossed. If this were the case, then natural selection would act with double strength; and those which were better adapted to survive both structurally and physiologically would certainly do so.]
[Footnote 63: Cases of this kind are referred to at p. 155. It must, however, be noted, that such sterility in first crosses appears to be equally rare between different species of the same genus as between individuals of the same species. Mules and other hybrids are freely produced between very distinct species, but are themselves infertile or quite sterile; and it is this infertility or sterility of the hybrids that is the characteristic--and was once thought to be the criterion--of species, not the sterility of their first crosses. Hence we should not expect to find any constant infertility in the first crosses between the distinct strains or varieties that formed the starting-point of new species, but only a slight amount of infertility in their mongrel offspring. It follows, that Mr. Romanes' theory of _Physiological Selection_--which assumes sterility or infertility between first crosses as the fundamental fact in the origin of species--does not accord with the general phenomena of hybridism in nature.]
[Footnote 64: The exact number is 1219.51, but the fractions are omitted for clearness.]
