Malthus on Population, that Natural Selection was the inevitable result of
the rapid increase of all organic beings; for I was prepared to appreciate
the struggle for existence by having long studied the habits of animals.

Before visiting the Galapagos I had collected many animals whilst
travelling from north to south on both sides of America, and everywhere,
under conditions of life as different as it is possible to conceive,
American forms were met with--species replacing species of the same
peculiar genera. Thus it was when the Cordilleras were ascended, or the
thick tropical forests penetrated, or the fresh waters of America searched.
Subsequently I visited other countries, which in all their conditions of
life were incomparably more like parts of South America, than the different
parts of that continent are to each other; yet in these countries, as in
Australia or Southern Africa, the traveller cannot fail to be struck with
the entire difference of their productions. Again the reflection was forced
on me that community of descent from the early inhabitants of South America
would alone explain the wide prevalence of American types throughout that
immense area.

To exhume with one's own hands the bones of extinct and gigantic quadrupeds
brings the whole question of the succession of species vividly before one's
mind; and I found in South America great pieces of tesselated armour
exactly like, but on a magnificent scale, that covering the pigmy
armadillo; I had found great teeth like those of the living sloth, and
bones like those of the cavy. An analogous succession of allied forms had
been previously observed in Australia. Here then we see the prevalence, as
if by descent, in time as in space, of the same types in the same areas;
and in neither the case does the similarity of the conditions by any means
seem sufficient to account for the similarity of the forms of life. It is
notorious that the fossil remains of closely consecutive formations are