In dealing with this subject in his “Autobiography,” Darwin tells us of his reflections whilst on the voyage of the _Beagle_, and here mentions another observation which deeply impressed him in addition to those which he again repeats, on the relation between the living and the dead in the same area and on the productions of the Galapagos Archipelago--viz. “the manner in which closely allied animals replace one another in proceeding southwards over the continent” (of South America). On the theory of separate creation the existence of such representative species received no explanation, although it became perfectly intelligible on the theory that a single species may be modified into distinct, although nearly related, species in the course of its range over a wide geographical area. Here, too, the evidence is in favour of evolution simply, and does not point to any cause of evolution. He also implies that even at this time he regarded the beautiful adaptations or contrivances of nature by which organisms are fitted 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”--as the most striking and important phenomena of the organic world, and the one great difficulty in the path of any naturalist who should attempt to supply a motive force for evolution. And he regarded the previous attempts at an explanation--the direct action of surroundings and the will of the organism--as inadequate because they could not account for such adaptations. Therefore being convinced of evolution, but as yet unprovided with a motive cause which in any way satisfied him, he began in July, 1837, shortly after his return home from the _Beagle_, to collect all facts which bore upon the modifications which man has induced in the animals and plants which he has subjugated, following, as he tells us, the example of Lyell in geology. He goes on to say in his “Autobiography”:-- “I soon perceived that selection was the key-stone 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.” [Sidenote: COLLECTION OF NOTES.]