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.' His first note-book for the accumulation of facts bearing on the question was opened in July 1837, and from that date he continued to gather them '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.' He soon perceived that selection was the secret of success in the artificial production of the useful varieties of plants and animals. But how this principle, so fertile in results when employed by man, could be applied in explanation of Nature's operations, remained a mystery to him until in October 1838, when, happening to read for amusement Malthus' book _On the Principle of Population_, he found at last a theory with which to work. With this guiding principle he instituted a laborious investigation on the breeding of pigeons, and experiments on the flotation of eggs, the vitality of seeds, and other questions, the solution of which seemed desirable as his researches advanced. He says himself that, to avoid prejudice in favour of his own views, he refrained for some time from writing even the briefest sketch of the theory he had formed, and that it was not until June, 1842, that he allowed himself the satisfaction of writing a very brief pencil abstract in thirty-five pages, which two years afterwards he enlarged to 230 pages, and had fairly copied out. This precious manuscript was the germ of the _Origin of Species_.