206 LAWS OF VAKIATION. [Chap. V. 



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 con- 

 stitution 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 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 beneficial differences 

 which has given rise to all the more important 

 modifications of structure in relation to the habits of 

 each species. 



