136 UNUSUALLY DEVELOPED PARTS 



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 geologi- 

 cal period. An extraordinary amount of modification im- 

 plies an unusually large and long-continued amount of 

 variability, which has continually been accumulated by nat- 

 ural selection for the benefit of the species. Hut 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 natural selec- 

 tion 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, nas 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 

 manner and degree, and by the continued rejection of those 

 tending to revert to a former and less modified condition. 



SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC 



CHARACTERS. 



The principle discussed under the last heading may be 

 applied to our present subject. It is notorious that specific 

 characters are more variable than generic. To explain by a 

 simple example what is meant : if in a large genus of plants 

 some species had blue flowers and some had red, the color 

 would be only a specific character, and no one would be sur- 



