Development of his Great Tlieory. 141 



was also impressed with the manner " in which 

 closely allied animals replaced each other in proceed- 

 ing southwards over the continent ; and thirdly, by 

 the South American character of most of the produc- 

 tions of the Galapagos archipelago, and more espe- 

 cially by the manner in which they differ slightly on 

 each island of the group ; none of the islands appear- 

 ing to be very ancient in a geological sense." 



The conclusions which he deduced from these 

 facts were that species became modified ; in other 

 words, their environment or conditions of life pro- 

 duced changes and modifications. This idea created 

 a profound and lasting impression ; as he says, the 

 subject haunted him, and he naturally began to look 

 for other facts bearing upon the same line of thought. 

 Upon his return to England he commenced a sys- 

 tematic collection of data referring to this subject. 

 He read with great care monographs, papers, and 

 reports. He interviewed people of all grades and 

 professions, sent out printed slips with questions and 

 answers, and rapidly accumulated a mass of facts that 

 would have appalled one less enthusiastic. The idea 

 was not yet fully developed ; he was still groping in 

 the dark when " Malthus on Population " fell in his 

 way, and wais read simply as a recreation. This was 

 in 1838, fifteen months after he began his collection 

 of facts, and from this source he obtained the idea 

 that in the struggle for existence between various 

 forms, " favourable variations tend to be preserved, 

 and unfavourable ones to be destroyed. The result 

 of this would be the formation of a new species." 



The idea must have come to him like a sudden 



