Origin of Species Variorum

in the carriage and tail of .. fantails, &c., these being the points now mainly attended to by English fanciers. Even in .. sub-breeds, as in that of the short-faced tumbler, it is notoriously difficult to breed .. nearly perfect birds, some frequently departing widely from the standard. There may truly be said to be a constant struggle going on between, on the one hand, the tendency to reversion to a less perfect state, as well as an innate tendency to further variability, and, on the other hand, the power of steady selection to keep the breed true. In the long run 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, ... much variability in the parts undergoing modification may always be expected. It further deserves notice that characters, modified through selection by man, are sometimes transmitted, 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 us turn to nature. When a part has been developed in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modification since the period when the species branched off from the common progenitor of the genus. This period will seldom be remote in any extreme degree, as species .. rarely endure for more than one geological period. An extraordinary amount of modification implies an unusually large and long-continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species. But as the variability of the extraordinarily developed part or organ has been so great and long-continued within a period not exces- sively remote,