transmutation of species. During the voyage of the _Beagle_ I had been
deeply impressed by discovering in the Pampean formation great fossil
animals covered with armour like that on the existing armadillos;
secondly, by the manner in which closely allied animals replace one
another in proceeding southwards over the continent; and, thirdly, by
the South American character of most of the productions of the Galapagos
Archipelago, and more especially by the manner in which these differ
slightly on each island of the group, none of these islands appearing to
be very ancient in a geological sense.

It was evident that such facts as these, as well as many others, could
only be explained on the supposition that species gradually become
modified; and the subject haunted me. But it was equally evident that
neither the action of the surrounding conditions, nor the will of the
organisms (especially in the case of plants) could account for the
innumerable cases in which organisms of every kind are beautifully
adapted to their habits of life--for instance, a woodpecker or a
tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I
had always been much struck by such adaptations, and until these could
be explained it seemed to me almost useless to endeavour to prove by
indirect evidence that species have been modified.

After my return to England it appeared to me that by following the
example of Lyell in geology,[2] and by collecting all facts that bore in
any way on the variation of animals and plants under domestication and
nature, some light might perhaps be thrown on the whole subject. My
first note-book was opened in July, 1837. I worked on true Baconian
principles, and without any theory collected facts on a wholesale scale,
more especially with respect to domesticated productions, by printed
enquiries, by conversation with skilful breeders and gardeners and by
extensive reading. When I see the list of books of all kinds which I
read and abstracted, including whole series of journals and
translations, I am surprised at my industry. I soon perceived that
selection was the keystone of man's success in making useful races of
animals and plants. But how selection could be applied to organisms
living in a state of nature remained for some time a mystery to me.

In October, 1838, that is fifteen months after I had begun my systematic
enquiry, I happened to read for amusement "Malthus on Population," and
being well pr