LAWS OF VARIATION 139 



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. 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 species branched off from the common 

 progenitor of the genus. This period will seldom be 

 remote in any extreme degree, as species very rarely 

 endure for more than one geological period. An extra- 

 ordinary 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 exces- 

 sively remote, we might, as a general rule, expect still 

 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 can see no 

 reason to doubt. Hence when an organ, however 

 abnormal it may be, has been transmitted in approxi- 

 mately the same condition to many modified descend- 

 ants, 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 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 continued selection of the individuals varying in 

 the required manner and degree, and by the continued 



