During the voyage of the Beagle I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos; secondly, by the manner in which closely allied animals replace one another in proceeding southwards over the continent; and thirdly, by the South American character of most of the productions of the Galapagos Archipelago, and more especially by the manner in which they differ slightly on each island of the group, none of the islands appearing to be very ancient in a geological sense. It was evident that such facts as these, as well as many others, could only be explained on the supposition that species gradually became modified; and the subject haunted me. But it was equally evident that "none of the evolutionary theories then current in the scientific world" could account for the innumerable cases in which organisms of every kind are beautifully adapted to their habits of life.... I had always been much struck by such adaptations, and until these could be explained, it seemed to me almost useless to endeavour to prove by indirect evidence that species have been modified.... In October, 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on, from long-continued observations of the habits of plants and animals, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones destroyed. The result of this would be the formation of new species. Shortly after his return he settled in London, prepared his journal and manuscripts of observations for publication, and opened, he says, under date of July, 1837, "my first note-book for facts in relation to the origin of species, about which I had long reflected, and never ceased working for the next twenty years." He acted for two years as one of the honorary secretaries of the Geological Society, which brought him into close relations with Lyell, and, as his health then allowed him to go into society, he saw a good deal of prominent literary and scientific contemporaries.