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Show 422 CONCLUDING REUARKS. CrrAP. xxvm. in the course of time become similarly modified. But a far commoner result would be, that one . bird would vary in one way and another bird in another way; one would be born with a little longer beak, and another with a shorter beak ; one would gain some black feathen~, another some white or red feathers. And as these birds would be continually intercrossing, the final result would be a body of individuals differing from each other slightly in many ways, yet far more than did the original roc~kpigeons. But there would not be the least tendency to the formation of distinct breeds. If two separate lots of pigeons were to be treated in tho manner just described, ono in England and the other in a . tropical country, the two lots being supplied with different food, would they, after many generations had passed, differ? When we reflect on the cases given in the twenty-third chapter, and on such facts as the difference in former times between the breeds of cattle, sheep, &c., in almost every district of Europe, we are strongly inclined to admit that the two lots would bo differently modified through the influence of climate and food. But the evidence on the definite action of changed conc1itions is in most cases insufficient; and, with respect to pi goons, I have had the opportunity of examining a large collection of domesticated birds, sent to me by Sir vV. Elliot from India, and they varied in a remarkably similar manner with our European birds. If two distinct breeds were to be confined together in equal numbers, there is reason to suspect that they would to a certain extent prefer pairing with their own kind; but they would likewise intercross. From the greater vigour and fertility of the crossed offspring, the whole body would by this means become interblended sooner than would otherwise have occurred. From certain breeds being prepotent over others, it does not follow that the interblended progeny would be strictly intermediate in character. I have, also, proved that the act of crossing in itself gives a strong tendency to reversion, so that the crossed offspring would tend to revert to the state of the aboriginal rock-pigeon. In the course of time they would probably be not much more heterogeneous in character than in our first case, when birds of the same breed were confined together. CHAP. XXVIII. CO~CLUDING REMARKS. 423 I have just said that the crossed offspring would gain in vigour and fertility. From the facts given in the seventeenth chapter there can be no doubt of this ; and there can be little doubt, though the evidence on this head is not so easily acquired, that long-continned close interbreeding leads to evil results. vVith hermaphrodites of all kinds, if the sexual elements of the same individual habitually acted on each other, the closest possible interbreeding would be perpetual. Therefore we should bear in mind that with all hermaphrodite animals, as far as I can learn, their structure permits and frequently necessitates a cross with a distinct individual. With hermaphrodite plants we incessantly meet with elaborate and perfect contrivances for this same end. It is no exaggeration to assert that, if the use of the talons and tusks of a carnivorous animal, or the use of the viscid threads of a spider's web, or of the plumes and hooks on a seed may be safely inferred from their structure, we may with ·equal safety infer that many flowers are constructed for the express purpose of ensuring a cross with a distinct plant. From these various considerations, the conclusion arrived at in the chapter just referred to-namely, that great good of some kind is derived from the sexual concourse of distinct individuals-must be admitted. To return to our illustration: we have hitherto assumed that the bll.·cls were kept down to the same number by indiscriminate slaughter; but if the least choice be permitted in their preservation and slaughter, the whole result will be changed. Should the owner observe any slight variation in one of his birds, and wish to obtain a breed thus characterised, he would succeed in a surprising! y short time by carefully selecting and pairing the young. As any part which has once varied generally goes on v.arying in the same direction, it is easy, by continually preservmg the most strongly marked individuals, to increase the amount of difference up to a high, predetermined standard of excellence. This is methodical selection. If the owne1~ of the aviary, without any thought of making a new breed, srmply admired, for instance, short-beaked more than long-beaked birds, he would, when he had to reduce the number, generally kill the latter; and there can be no doubt that he would thus in the course of time sensibly modify his |