RISE OF EVOLUTIONARY THOUGHT 427 
his book on the Origin of Species until he had worked on 
his theory twenty-two years. The circumstances that Icd 
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, al- 
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 Malihus 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 of 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 bricfest 
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 
