| OCR Text |
Show 152 LAWS OF VARIATION. CHAP. v. presumption is that it is of h!gh ~mpo~ce ~ that species; nevertheless the part In thi.s case IS eminently liable to variation. Why should this be so ? On the view that each species has been independently created, with all its parts as we now see them, I can see ~o explanation. But on the view that groups of species have descended from other species, and have been modified through natural selection, I think we can obtain some light. In our domestic animals, if an~r part, or the whole animal, be neglected and no selection be applied, that part (for instance, the comb in the Dorking fowl) or the whole breed will cease to have a nearly uniform character. The breed will then be said to have degenerated. In rudimentary organs, and in thm;e which have been but little specialised for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel natural case; for in such cases natural selection either has not or cannot come into full play, and thus the organisation is left in a fluctuating condition. But what here more especially concerns us is, that in our domestic animals those points, which at the present time are undergoing rapid change by continued selection, are also eminently liable to variation. Look at the breeds of the pigeon ; see what a prodigious amount of difference there is in the beak of the different tumblers, in the beak and wattle of the different carriers, in the carriage and tail of our fantails, &c., these being the points now mainly attended to by English fanciers. Even in the sub-breeds, as in the shortfaced tumbler, it is notoriously difficult to breed them nearly to perfection, and frequently individuals are born which depart widely from the standard. There may be truly said to be a constant struggle going on between, on the one hand, the tendency to reversion to a less modified state, as well as an innate tendency to further CHAP. V. LAWS OF VARIATION. 153 variability of all kinds, and, on the other hand th power of steady sel~ction to keep the breed true~ 1: the long ru? selection gains the day, and we do not expect to fail so far as to breed a bird as coarse as a common tumbler from a good short-faced strain. But as long as selection is rapidly going on, there rna always be ex~ected to ?e much variability in the stru! tur~ undergoing modification. It further deserves notice. that these variable characters, produced by man's selection, sometimes become attached, from causes quite unknown to us, more to one sex than to the other, generally to the male sex, as with the wattle of carriers and the enlarged crop of pouters. Now let ~s turn to nature. When a part has been deve:oped In an e~traordinary manner in any one spemes, compared with the other species of the same genus, we m.ay conclude that this part has undergone an . extraordmary amount of modification, since the period :Vhen the species branched off from the common progerut?r of the genus. This . period will seldom be remote In any extreme degre~,. as_ species very rarely end~re for more than one geological period. An extraordinary amount of modification implies an unusually large an~ long-continued. amount of variability, which has continually been accum-ulated by natural selection for the benefit of the species. But as the variability of the extraordinarily-developed part or organ has been so ~reat and long-continued within a period not excesSIVely remote, we might, as a general rule, expect still to find more :ari~bility i.n such parts than in other parts of the orgarusation, which have remained for a much l~nger period nearly constant. And this, I am conVInced, is the case. That the struggle between natural s~lection on the one hand, and the tendency to reverSion and variability on the other hand, will in the H3 |
