al, in which he purposed to record all
facts that came to him which seemed to have any bearing on the moot
point of the doctrine of transmutation of species. Four or five years
earlier, during the course of that famous trip around the world with
Admiral Fitzroy, as naturalist to the Beagle, Darwin had made the
personal observations which first tended to shake his belief of the
fixity of species. In South America, in the Pampean formation, he had
discovered "great fossil animals covered with armor like that on the
existing armadillos," and had been struck with this similarity of type
between ancient and existing faunas of the same region. He was also
greatly impressed by the manner in which closely related species of
animals were observed to replace one another as he proceeded southward
over the continent; and "by the South-American character of most of the
productions of the Galapagos Archipelago, and more especially by the
manner in which they differ slightly on each island of the group, none
of the islands appearing to be very ancient in a geological sense."

At first the full force of these observations did not strike him; for,
under sway of Lyell's geological conceptions, he tentatively explained
the relative absence of life on one of the Galapagos Islands by
suggesting that perhaps no species had been created since that island
arose. But gradually it dawned upon him that such facts as he had
observed "could only be explained on the supposition that species
gradually become modified." From then on, as he afterwards asserted, the
subject haunted him; hence the journal of 1837.

It will thus be seen that the idea of the variability of species came to
Charles Darwin as an inference from personal observations in the field,
not as a thought borrowed from books. He had, of course, read the works
of his grandfather much earlier in life, but the arguments of Zoonomia
and The Temple of Nature had not served in the least to weaken his
acceptance of the current belief in fixity of species. Nor had he been
more impressed with the doctrine of Lamarck, so closely similar to that
of his grandfather. Indeed, even after his South-American experience had
aroused him to a new point of view he was still unable to see anything
of value in these earlier attempts at an explanation of the variation
of species. In opening his journal, therefore, he had no preconceived
notion of upholding the views of these or any other makers of
hypotheses