440 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
to change his previous views in regard to the nature and origin of 
species. 
When he first began his explorations in South America he was, 
as he expressly says, still completely under the influence of the 
dogma of the creation of species once for all, and their immutability, 
and he regarded it as unassailable. But very soon he was struck 
by certain facts which seemed to him difficult to reconcile with this 
dogma, and these increased in number in the course of his journey, 
till finally they led him to the conviction that the old position was 
untenable, and that the organic world had not been created immu- 
table, but had slowly evolved. 
T select two of these phenomena, first, the occurrence of the fossil 
remains of gigantic mammals in the diluvial strata of the great 
plains of La Plata and Patagonia. Darwin found a gigantic arma- 
dillo (Dasypus gigas), and he was led to ask how it happened that 
small armadillos now live in South America, whereas they do not 
occur, either living or fossil, anywhere else in the world. The answer 
was easy, if it was possible to assume that the present-day species 
were descended from the diluvial forms, or from other smaller, still 
undiscovered forms from the same period. But he was especially 
impressed by the fauna and flora of the Galapagos Islands, which 
lie under the equator, 500 nautical miles to the west of the South 
American coast. 
On these isolated and comparatively barren volcanic islands there 
live many animals which could not fail to arrest the attention of the 
-naturalist—land birds which are like those of the neighboring conti- 
nent, and are of purely American type, yet are not identical but 
closely related species. Most of them are so-called “ endemic ” 
species, that is, species which occur in no other part of the world. 
This was striking enough, but the matter proved even more remark- 
able on closer investigation, for several of the fifteen islands of which 
the archipelago consists possess species of the same genus peculiar 
to themselves—mocking thrushes, for instance, which are represented 
in the other islands by similar but not identical species. 
What inference is possible from these facts except that, at some 
earlier period, bird migrants from the neighboring continent had 
landed on these volcanic islands, and in the course of thousands of 
years had varied, that is to say, had become distinct species on each 
island ? 
These and other phenomena aroused in Darwin’s mind the idea 
of evolution, and he resolved to devote his attention to this problem 
after he returned home, for he was persuaded that he could attain to 
certainty in regard to it by patiently collecting facts. Thus he set 
himself the task of his life. It may be well to inquire here whether, 
or to what extent, Darwin had taken over the idea of evolution from 
