At this time he had the conception--as we see in the succeeding extracts from his Diary--of species being so constituted that they must give rise to other species; or, if not, that they must die out, just as an individual dies unrepresented if it has no offspring; that change--and evidently change in some fixed direction--or extinction, is inevitable in the history of a species after a certain period of time. With this view, which presented much resemblance to that of the author of the “Vestiges,” and which seemed uppermost in his mind at this time, there are traces of others. Thus in one extract the “wish of parents” was thought of as a very doubtful explanation of adaptation, while in another we meet a tolerably clear indication of natural selection, a variety which is not well adapted being doomed to extinction, while a favourable one is perpetuated, the death of a species being regarded as “a consequence ... of non-adaptation of circumstances.” It seems certain that for fifteen months after July, 1837, he was keenly considering the various causes of evolution which were suggested to him by the facts of nature, and that some general idea of natural selection presented itself to him at times, although without any of the force and importance it assumed in his mind at a later time. In October, 1838, he read “Malthus on Population,” and as he says:-- “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 favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here then I had a theory by which to work.” [Sidenote: SKETCH OF THE “ORIGIN.”] In June, 1842, he wrote a brief account of the theory, occupying thirty-five pages. In Lyell’s and Hooker’s introduction to the joint paper by Darwin and Wallace in the Linnean Society’s Journal (1858) it is stated that the first sketch was made in 1839, but Francis Darwin shows (“Life and Letters,” 1887, Vol. II. pp. 11, 12) that in all probability this is an error--a note of Darwin’s referring to the first complete grasp of the theory after reading Malthus, being mistaken for a reference to the first written account. In 1844 the sketch was enlarged to a written essay occupying 231 pages folio--“a surprisingly complete presentation of the argument afterwards familiar to us in the ‘Origin of Species’” published fifteen years later. Professor Huxley, after reading this essay, observed that “much more weight is attached to the influence of external conditions in producing variation, and to the inheritance of acquired habits than in the ‘Origin,’” while Professor Newton pointed out that the remarks on the migration of birds anticipate the views of later writers.[C]