2io THE DARWINIAN THEORY 



in the Galapagos Islands. The problems which 

 occurred to him were : " Why are the animals 

 of the latest geological epoch in South America 

 similar in fades to those which exist in the same 

 region at the present day, and yet specifically and 

 generically distinct ? " And, " Why are the animals 

 and plants of the Galapagos Archipelago so like 

 those of South America, and yet different from 

 them ? Why are those of the several islets more or 

 less different from one another ? " 



These problems were only explicable on the 

 assumption of modification, and in order to explain 

 the cause of these modifications he turned to the 

 only certainly known examples of descent with 

 modification — viz., those presented by domestic 

 animals and cultivated plants. The details of these 

 he worked up and experimented upon in a much 

 more thorough manner than his predecessors, 

 especially in regard to pigeons. He soon perceived 

 "that selection was the keystone of the main 

 success in making useful races of animals and 

 plants " ; but says : " how selection could be applied 

 to organisms living in a state of nature remained for 

 some time a mystery to me." 



"In October 1838," he writes, "that is, fifteen 

 months after I had begun my systematic inquiry, I 

 happened to read for amusement, ' Malthus on 

 Population,' and being well prepared to appreciate 

 the struggle for existence which everywhere goes on 

 from long-continued observations of the habits of 

 animals and plants, it at once struck me that under 

 these circumstances favourable variations would 



