Darwin, writing in his Autobiography about natural selection:
In October, 1838, that is, 15 months after I had begun my systematic enquiry, … it at once struck me that under these circumstances [struggle for existence, as in Malthus] favorable variations would tend to be preserved, and unfavorable ones to be destroyed … 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”