LAWS OF VARIATION 135 



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 character to the 

 organ, in however extraordinary a manner it may have been de- 

 veloped. 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 oc- 

 casionally 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 modifications will 

 add to the beautiful and harmonious diversity of nature. 



Whatever the cause may be of each slight difference between the 

 offspring and their parents — and a cause for each must exist — we 

 have reason to believe that it is the steady accumulation of bene- 

 ficial differences which has given rise to all the more important 

 modifications of structure in relation to the habits of each species. 



