revised his notes, and
reconsidered the impressions made on his mind, the 'vague doubts' he had
entertained, from time to time, concerning the immutability of species,
would come back to him with new force and cumulative effect. 'I then
saw,' he says, 'how many facts indicated the common descent of species,'
and further, 'It occurred to me in 1837, that something might perhaps be
made out on this question by patiently accumulating and reflecting on
all sorts of facts which could possibly have any bearing on it.' In July
of that year, he opened his first note-book on the subject[106]--the
note-books being soon replaced by a series of portfolios, in which
extracts from the various works he read, facts obtained by
correspondence, the records of experiments and observation, and ideas
suggested by constant meditation were slowly accumulated for twenty
years. Mr Francis Darwin has published a series of extracts from the
note-book of 1837, which amply prove that by this time Charles Darwin
had become 'a convinced evolutionist[107].'

Fifteen months after this 'systematic enquiry' began, Darwin happened to
read the celebrated work of Malthus _On Population_, for amusement, and
this served as a spark falling on a long prepared train of thought. The
idea that as animals and plants multiply in geometrical progression,
while the supplies of food and space to be occupied remain nearly
constant, and that this must lead to a 'struggle for existence' of the
most desperate kind, was by no means new to Darwin, for the elder De
Candolle, Lyell and others had enlarged upon it; yet the facts with
regard to the human race, so strikingly presented by Malthus, brought
the whole question with such vividness before him that the idea of
'Natural Selection' flashed upon Darwin's mind. This hypothesis cannot
be better or more succinctly stated than in Huxley's words.

'All _species_ have been produced by the development of
_varieties_ from common stocks: by the conversion of these,
first into _permanent races_ and then into _new species_, by the
process of _natural selection_, which process is essentially
identical with that artificial selection by which man has
originated the races of domestic animals--the _struggle for
existence_ taking the place of man, and exerting, in the case of
natural selection, that selective action which he performs in
artificial selection[108].'

With characteristic caution, Darwin