144 UNU8UALLT DEVELOPED PARTS 



the other hand, the power of steady selection to keep the 

 breed true. In the Jong 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 modi- 

 fication 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 extrerne 

 degree, as species rarely endure for more than one geolog- 

 ical 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 excessively remote, we might, as a general rule, still 

 expect to find more variability in such parts than in 

 other parts of the organization which have remained for 

 a much longer period nearly constant. And this, I am 

 convinced, is the case. That the struggle between nat- 

 ural selection on the one hand, and the tendency to rever- 

 sion 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 in this case the 

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

 tinued selection of the individuals varying in the required 



