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:—
"I had previously read the Zoonomia of my grandfather, in which similar views are maintained, but without producing any effect on me. Nevertheless, it is probable that the hearing rather early in life such views maintained and praised, may have favoured my upholding them under a different form in my Origin of Species. At this time I admired greatly the Zoonomia; but on reading it a second time after an interval of ten or fifteen years, I was much disappointed; the proportion of speculation being so large to the facts given."
Mr. Huxley has well said (Obituary Notice, p. ii.): "Erasmus Darwin, was in fact an anticipator of Lamarck, and not of Charles Darwin; there is no trace in his works of the conception by the addition of which his grandson metamorphosed the theory of evolution as applied to living things, and gave it a new foundation."
On the whole it seems to me that the effect on his mind of the earlier evolutionists was inappreciable, and as far as concerns the history of the Origin of the Species, it is of no particular importance, because, as before said, evolution made no progress in his mind until the cause of modification was conceivable.
I think Mr. Huxley is right in saying[128] that "it is hardly too much to say that Darwin's greatest work is the outcome of the unflinching application to biology of the leading idea, and the method applied in the Principles to Geology." Mr. Huxley has elsewhere[129] admirably expressed the bearing of Lyell's work in this connection:—
"I cannot but believe that Lyell, for others, as for myself, was the chief agent in smoothing the road for Darwin. For consistent uniformitarianism postulates evolution as much in the organic as in the inorganic world. The origin of a new species by other than ordinary agencies would be a vastly greater 'catastrophe' than any of those which Lyell successfully eliminated from sober geological speculation....