In the long run selection gains the day, and we do not expect to fail so completely as to breed a bird as coarse as a common tumbler pigeon 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.
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 several 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.