90 THE THEORIES OF EVOLUTION 



with the help of selection fostering favourable vari- 

 ations. 



Certain cases of parallel_adaptation are even more 

 difficr.lt to explain; cases in which variations must 

 affect two different individuals, when for instance the 

 reproductive organs of the male must fit very exactly 

 those of the female to make fertilisation possible; 

 peases in which the organs and instincts of an insect in- 

 suring the fertilisation of a plant must correspond 

 closely to the plant's mode of reproduction, and other 

 cases of mutual adaptation. 



Upon the publication of the "Origin of Species " 

 many naturalists formulated similar criticisms, men- 

 tioning extremely complex organs, like the eye of the 

 vertebrates which presupposes many co-ordinated va- 

 riations. We remember what a general answer Dar- 

 win gave to them, an answer addressed only to the 

 partisans of the invariability of species. Wallace 

 reasons differently and contends that those parallel 



(changes can very well be due to natural selection, 

 since we see them produced by artificial selection. 

 And thus we are led to discuss the analogy between 

 these two factors of evolution. 



This analogy, a fundamental idea of Darwin's doc- 

 trine which he admitted without discussion, did not 

 meet with any opposition during the first years which 

 followed the publication of his book. It was not 

 until many years later, in 1893, that Spencer attacked 



