15.4 UNUSUALLY DEVELOPED PARTS Cnxr. V. 



the tendency to reversion to a less perfect state, as well as an 

 innate tendency to fnrtlier 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 faU 

 so far as to breed a bird as cf)arse as a common tumbler from a 

 good short-faced strain. Bnt as long as selection is rapidly 

 going on, nnich variability in the parts \mdergoing modification 

 may always he, expected. It further deserves notice tliat char- 

 acters, modified through selection by man, are sometimes trans- 

 mitted, 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. AYlien a part has been devel- 

 oped 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 modifi- 

 cation since the period when the species branchetl off from the 

 connnon 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 amovmt 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 

 ■];)een so great and long-continued within a period not exces- 

 sively remote, Ave might, as a general rule, still expect to find 

 more A-ariability in such parts than in otlier parts of the organi- 

 zation which have remained for a much longer period nearly 

 constant. And this, I am convinced, is the case. Tliat 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 devel- 

 oped organs may be made constant, I see no reason to doubt. 

 Ilence, when an organ, however abnormal it may be, has been 

 transmitted in approximately the same condition to many modi- 

 fied descendants, as in the case of the wing of the bat, it must 

 have existed, according to my theory, for an immense period 

 in nearly the same state; and thus it comes to be no more vari- 

 al)le than any other structure. It is only in those cases in 

 wliich the modification has been comparatively recent and 

 extraordinarily great that we ought to find i\\o generative vari- 

 (ihllitij^ as it may be called, still present in a high degree. For 

 in this case tlie variability will seldom as yet have been fixed 



