148 , SUMMAitf. 



same genus. Any part or organ developed to an extraordi- 

 nary size or in an extraordinary manner, in comparison with 

 the same part or organ in the allied species, must have gone 

 through an extraordinary amount of modification since the 

 genus arose ; and thus we can understand why it should 

 often still be variable in a much higher degree than other 

 parts ; for variation is a long-continued and slow process, 

 and natural selection will in such cases not as yet have had 

 time to overcome the tendency to further variability and to 

 reversion to a less modified state. But when a species with 

 an extraordinarily developed organ has become the parent of 

 many modified descendants — which on our view must be a 

 very slow process, requiring a long lapse of time — in this 

 case, natural selection has succeeded in giving a fixed char- 

 acter to the organ, in however extraordinary a manner it 

 may have been developed. Species inheriting nearly the 

 same constitution from a common parent, and exposed to 

 similar influences, naturally tend to present analogous varia- 

 tions, or these same species may occasionally revert to some 

 of the characters of their ancient progenitors. Although 

 new and important modifications may not arise from rever- 

 sion and analogous variation, such modifications will add to 

 the beautiful and harmonious diversity of nature. 



Whatever the cause may be of each slight difference be- 

 tween the offspring and their parents — and a cause for each 

 must exist — we have reason to believe that it is the steady 

 accumulation of beneficial differences which has given rise 

 to all the more important modifications of structure in rela- 

 tion to the habits of each species. 



