RISE OF EVOLUTIONARY THOUGHT 435 



his book on the Origin of Species until he had worked on 

 his theory twenty-two years. The circumstances that led 

 to his publishing it when he did have already been indi- 

 cated. 



Parallelism in the Thought of Darwin and Wallace. 

 No one can read the letters of Darwin and Wallace explaining 

 how they arrived at their idea of natural selection without 

 marveling at the remarkable parallelism in the thought of the 

 two. It is a noteworthy circumstance that the idea of natural 

 selection came to both by the reading of the same book, Mal- 

 thus on Population. 



Darwin's statement of how he arrived at the concep- 

 tion of natural selection is as follows: "In October, 1838, 

 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 tend to be preserved and unfavourable ones 

 to be destroyed. The result o) this would be the formation 

 of new species. Here then I had at last got a theory by 

 which to work, but I was so anxious to avoid prejudice that 

 I determined not for some time to write even the briefest 

 sketch of it. In June, 1842, I first allowed myself the satis- 

 faction of writing a very brief abstract of my theory in pencil, 

 in thirty-five pages, and this was enlarged during the summer 

 of 1844 into one of 230 pages." 



And Wallace gives this account: "In February, 1858, I 

 was suffering from a rather severe attack of intermittent fever 

 at Ternate, in the Moluccas; and one day, while lying on 

 my bed during the cold fit, wrapped in blankets, though the 

 thermometer was at 88 Fahr., the problem again presented 

 itself to me, and something led me to think of the 'positive 



