CHAP. V.] HIGHLY VARIABLE. 113 



same sub-breed, as in that of the short-faced tumbfer, it is notori- 

 ously difficult to breed nearly perfect birds, many 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 

 new variations, and, on the other hand, the i>ower of steady selec- 

 tion 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 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 varia- 

 bility, which has continually been accumulated by natural selec- 

 tion 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 extra- 

 ordinarily great that we ought to find the generative variability, 

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

 case the variability will seldom as yet have been fixed by the con- 

 tinued selection of the individuals varying in the required manner 

 and degree, and by the continued rejection of those tending to 

 revert to a former and less-modified condition. 



