Chap. V.] UNUSUALLY DEVELOPED PARTS HIGHLY VARIABLE. 121 



the individuals of the same breed of the pigeon, and see what a 

 prodigious amount of difference there is in the beaks of tumblers, in 

 the beaks and wattle of carriers, in the carriage and tail of fantails, 

 &c, these being the points now mainly attended to by English 

 fanciers. Even in the same sub-breed, as in that of the short-faced 

 tumbler, it is notoriously difficult to breed nearly perfect birds, 

 many departing widely from the standard. There may truly be 

 6aid 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 new variations, 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 completely as to breed 

 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 ns 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 con- 

 tinually 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 excessively remote, we might, as a general rule, still expect to 

 find more variability in such parts than in other parts of the 

 organisation which have remained for a much longer period nearly 

 constant. And this, I am convinced, is the case. That the struggle 

 between natural selection on the one hand, and the tendency to 

 reversion and variability on the other hand, will in the course of 

 time cease ; and that the most abnormally developed organs may be 

 made constant, I see no reason to doubt. Hence, when an organ, 

 however abnormal it may be, has been transmitted in approximately 

 the same condition to many modified descendants, as in the case of 

 the wing of the bat, it must have existed, according to our theory, 

 for an immense period in nearly the same state ; and thus it has 

 come not to be more variable than any other structure. It is only 

 in those cases in which the modification has been comparatively 

 recent and extraordinarily great that we ought to find the generative, 

 variability, as it may be called, still present in a high degree. For 



