In the autobiographical chapter, my father has given an account of his share in this great work: the present chapter does little more than expand that story.
Two questions naturally occur to one: (1)--When and how did Darwin become convinced that species are mutable? How (that is to say) did he begin to believe in evolution. And (2)--When and how did he conceive the manner in which species are modified; when did he begin to believe in Natural Selection?
The first question is the more difficult of the two to answer. He has said in the _Autobiography_ (p. 39) that certain facts observed by him in South America seemed to be explicable only on the "supposition that species gradually become modified." He goes on to say that the subject "haunted him"; and I think it is especially worthy of note that this "haunting,"--this unsatisfied dwelling on the subject was connected with the desire to explain _how_ species can be modified. It was characteristic of him to feel, as he did, that it was "almost useless" to endeavour to prove the general truth of evolution, unless the cause of change could be discovered. I think that throughout his life the questions 1 and 2 were intimately,--perhaps unduly so, connected in his mind. It will be shown, however, that after the publication of the _Origin_, when his views were being weighed in the balance of scientific opinion, it was to the acceptance of Evolution not of Natural Selection that he attached importance.
An interesting letter (Feb. 24, 1877) to Dr. Otto Zacharias,[127] gives the same impression as the _Autobiography_:--
"When I was on board the _Beagle_ I believed in the permanence 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 immediately began to prepare my Journal for publication, and then saw how many facts indicated the common descent of species, so that in July, 1837, I opened a note-book to record any facts which might bear on the question. But I did not become convinced that species were mutable until, I think, two or three years had elapsed."
Two years bring us to 1839, at which date the idea of natural selection had already occurred to him--a fact which agrees with what has been said above. How far the idea that evolution is conceivable came to him from earlier writers it is not possible to say. He has recorded in the _Autobiography_ (p. 38) the "silent astonishment with which, about the year 1825, he heard Grant expound the Lamarckian philosophy." He goes on:--