214 ORGANIC EVOLUTION CONSIDERED 



duce sterile offspring that would have more highly 

 developed instincts. I have already dwelt upon this 

 subject at sufficient length. 



At the close of his chapter on Instinct, Mr. Darwin 

 says: " I do not pretend that the facts given in this 

 chapter strengthen in any great degree my theory, 

 but none of the cases of difficulty, to the best of my 

 judgment, annihilate it." * 



I will next consider briefly certain instincts of birds 

 which, it seems to me, could not have been evolved. 

 First, the instinct which causes the bird to incubate 

 its eggs. While the eggs of cold-blooded vertebrates 

 are hatched by the heat of the elements in which 

 they are deposited, the eggs of birds are generally de- 

 veloped by the heat of the mother's body. 



In passing from reptile to bird by evolution, it is 

 necessary to account for the changes in the instincts 

 of the bird as well as for its changes in structure. 



The act of incubation is one of self-denial, during 

 which the bird gives up its liberty for from two to 

 four weeks. How could the instinct that such an act 

 was necessary have been evolved? That the instinct 

 is necessary in order to develop the eggs of most birds 

 there can be no doubt, for if left simply to the ele- 

 ments they would perish. Did the instinct arise be- 

 fore the necessity for incubation was evolved, or vice 

 versa? If the instinct arose first, then it existed at a 

 time when there was no use for it, and the bird imposed 

 upon itself a useless task — a thing which we do not 

 find occurring among animals, for every existing 

 instinct serves some good purpose. 



Besides, we could not account for the preservation 

 of the instinct by natural selection unless it served a 

 good purpose. If the necessity for the instinct arose 

 before the instinct itself was evolved, then the species 

 would have perished for lack of the instinct. 



* Origin of Species, p. 231. 



