others, could
be explained only 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 endeavor 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, and by collecting all facts which 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
inquiries, 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
transactions, 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 inquiry, I happened to read for amusement Malthus's _Essay on
the Principle of Population_, and being well prepared to appreciate the
struggle for existence which everywhere goes on, from long-continued
observation of the habits of animals and plants, it at once struck me
that under these circumstances favorable variations would tend to be
preserved, and unfavorable ones to be destroyed. The result of this
would be the formation of new species. Here then I had at last got a
theory by which to work; but I was so anxious to avoid prejudice that I
determined not for some time to write even the briefest sketch of it. In
June, 1842, I first allowed myself the satisfaction of writing a very
brief abstract of my theory in pencil in thirty-fi