214 THE ORIGIN OF SPECIES 



plctely as to breed a bird as coarse as a common tum- 

 bler 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 spe- 

 cies, compared with the other species of the same genus, 

 we may conclude that this part has undergone an ex- 

 traordinary amount of modification since the period when 

 the several species branched off from the common pro- 

 genitor of the genus. This period will seldom be remote 

 in any extreme degree, as species rarely endure for more 

 than one geological period. An extra( rdinary amount 

 of modification implies an unusually large and long- 

 cortinued amount of variability, which has continually 

 been accumulated by natural selection for the benefit 

 of the species. But as the variability of the extraor- 

 dinarily 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 organ- 

 ization 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 'f 

 the other hand, will in the course of time cease; and 

 that the most abnormally developed organs may be made y 

 constant, I see no reason to doubt. Hence, when an 

 organ, however abnormal it may be, has been trans- 

 mitted 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, 



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