with his brother Erasmus in London, and,
after his brother's death, with his daughter, Mrs Litchfield. On these
occasions, it was his habit to arrange meetings with Huxley, to talk
over zoological questions, with Hooker, to discuss botanical problems,
and with Lyell to hold conversations on geology. After the death of
Lyell, Darwin, knowing my close intimacy with his friend during his
later years, used to ask me to meet him when he came to town, and "talk
geology." The "talks" took place sometimes at Jermyn Street Museum, at
other times in the Royal College of Science, South Kensington; but
more frequently, after having lunch with him, at his brother's or his
daughter's house. On several occasions, however, I had the pleasure of
visiting him at Down. In the postscript of a letter (of April 15, 1880)
arranging one of these visits, he writes: "Since poor, dear Lyell's
death, I rarely have the pleasure of geological talk with anyone.")

In one of the very interesting conversations which I had with Charles
Darwin during the last seven years of his life, he asked me in a very
pointed manner if I were able to recall the circumstances, accidental or
otherwise, which had led me to devote myself to geological studies. He
informed me that he was making similar inquiries of other friends, and I
gathered from what he said that he contemplated at that time a study
of the causes producing SCIENTIFIC BIAS in individual minds. I have
no means of knowing how far this project ever assumed anything like
concrete form, but certain it is that Darwin himself often indulged
in the processes of mental introspection and analysis; and he has
thus fortunately left us--in his fragments of autobiography and in his
correspondence--the materials from which may be reconstructed a fairly
complete history of his own mental development.

There are two perfectly distinct inquiries which we have to undertake
in connection with the development of Darwin's ideas on the subject of
evolution:

FIRST. How, when, and under what conditions was Darwin led to a
conviction that species were not immutable, but were derived from
pre-existing forms?

SECONDLY. By what lines of reasoning and research was he brought to
regard "natural selection" as a vera causa in the process of evolution?

It is the first of these inquiries which specially interests the
geologist; though geology undoubtedly played a part--and by no means an
insignificant part--in respect to the se