It was evident that such facts as these, as well as many 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-five pages; and this was enlarged during the summer of 1844 into one of two hundred thirty pages, which I had fairly copied out and still possess.