THE DOGMA OF EVOLUTION 



with atavism, or reversion to a distant ancestor, and 

 saw that a detailed study of the embryo was of im- 

 portance in problems of heredity. Thus, he quotes 

 from the collection of Hippocratic writings that, if 

 on succeeding days an egg of a setting of chickens is 

 opened and observed, we can learn the full history 

 of the growth of the embryo of the chick and will find 

 that the same process of development occurs in the 

 embryos of other animals. 



When he describes what he, himself, has observed 

 he is extraordinarily accurate, but, as he was forced 

 to depend also upon hearsay and on the observations 

 of others, there are many false and worthless state- 

 ments in his three great biological treatises. In com- 

 mon with the belief of his day, he thought many in- 

 sects were spontaneously generated from putrefying 

 earth and vegetable matter, and that others were 

 generated in the insides of animals out of the secre- 

 tions of their several organs. As he also expressly 

 states that each kind of animal is begotten only by 

 its own kind, he reconciles the two statements by sup- 

 posing that insects spontaneously generated are im- 

 perfect species and, when so generated, the adult 

 males and females produce offspring never identical 

 in shape with the parents but something imperfect. 

 For instance, lice, spontaneously arising from dust 

 afterwards produce nits; and flies produce grubs." 

 He thus could reconcile his belief that animals are 



^^Hist. Animal., V, i ; 539 b. 



