16 THE THEORY OF EVOLUTION 



plants of these islands are of distinctly American 

 type, yet most of the species are peculiar to the 

 group and each island has a surprising number of 

 species peculiar to itself. * Reviewing the facts here 

 given, one is astonished at the amount of creative 

 force if such an expression may be used displayed 

 on these small, barren, rocky islands; and still more 

 so at its diverse and yet analogous action on points 

 so near each other." 



The growth of his belief in evolution is thus de- 

 scribed by Darwin in a letter written in 1877: "When 

 I was on board the 'Beagle/ I believed in the per- 

 manence of species, but, as far as I can remember, 

 vague doubts occasionally flitted across my mind. 

 On my return home in the autumn of 1836, I im- 

 mediately began to prepare my journal for publica- 

 tion, and then saw how many facts indicated the 

 common descent of species. . . . But I did not 

 become convinced that species were mutable until 

 I think two or three years had elapsed." Even after 

 he had gained his conviction of the . mutability of 

 species, he could imagine no adequate cause of the 

 change and, consciously adopting the methods which 

 Lyell had used in Geology, he made a great collec- 

 tion of facts, gathered from breeders, fanciers, gar- 

 deners, as well as from his own observations, bearing 

 on the changes and modifications which men had 

 brought about in the various races and breeds of 

 domesticated animals and plants. 'I soon perceived 

 that selection was the key-stone of man's success 



