n character of
South American fossils," which suggests at least that the impression
made in 1832 required reinforcement before a really powerful effect was
produced.

We may therefore conclude, I think, that the evolutionary current in my
father's thoughts had continued to increase in force from 1832 onwards,
being especially reinforced at the Galapagos in 1835 and again in 1837
when he was overhauling the results, mental and material, of his
travels. And that when the above record in the Pocket Book was made he
unconsciously minimised the earlier beginnings of his theorisings, and
laid more stress on the recent thoughts which were naturally more vivid
to him. In his letter{11} to Otto Zacharias (1877) he wrote, "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." This again is evidence in favour of the view
that the later growths of his theory were the essentially important
parts of its development.

{11} F. Darwin's _Life of Charles Darwin_ (in one volume), 1892, p.
166.

In the same letter to Zacharias he says, "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." Unless Prof.
Judd and I are altogether wrong in believing that late or early in the
voyage (it matters little which) a definite approach was made to the
evolutionary standpoint, we must suppose that in 40 years such advance
had shrunk in his recollection to the dimensions of "vague doubts." The
letter to Zacharias shows I think some forgetting of the past where the
author says, "But I did not become convinced that species were mutable
until, I think, two or three years had elapsed." It is impossible to
reconcile this with the contents of the evolutionary Note Book of 1837.
I have no doubt that in his retrospect he felt that he had not been
"convinced that species were mutable" until he had gained a clear
conception of the mechanism of natural selection, _i.e._ in 1838-9.

But even on this last date there is some room, not for doubt, but for
surprise. The passage in the Autobiography{12} is quite clear, namely
that in October 1838 he read Malthus's _Essay on the principle of
Population_ and "being well prepared to appreciate the struggle for
existence ..., it at once struck me that under these circumstances
favourable variations