SUMMARY. 157 



varieties. Secondary sexual characters are highly variable, 

 and such characters differ much in the species of the same 

 group. Variability in the same parts of the organization 

 has generally been taken advantage of in giving secondary 

 sexual differences to the two sexes of the same species, and 

 specific differences to the several species of the same genus. 

 Any part or organ developed to an extraordinary sizu or in 

 an extraordinary manner, in comparison with the same 

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

 an extraordinary amount of modilication since the geims 

 arose; and thus we can understand why it slioukl 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 — wliich 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 character 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 variations, or these same species may occasionally 

 revert to some of the characters of their ancient progeni- 

 tors. Although new^ and important modifications may not 

 arise from reversion and analogous variation, such modi- 

 fications will add to the beautiful and harmonious diver- 

 sity 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 difi'crences which has 

 given rise to all the more important modifications of struc- 

 ture in relation to the habits of each species. 



