The first indication of these thoughts in any of his published letters is in one to his cousin Fox written in June, 1838, in which, after alluding to some questions he had previously asked about the crossing of animals, he says, “It is my prime hobby, and I really think some day I shall be able to do something in that most intricate subject--species and varieties.” He is rather more definite in a letter to Sir Charles Lyell, written September 13th in the same year:-- “I have lately been sadly tempted to be idle--that is, as far as pure geology is concerned--by the delightful number of new views which have been coming in thickly and steadily,--on the classification and affinities and instincts of animals--bearing on the question of species. Note-book after note-book has been filled with facts which begin to group themselves _clearly_ under sub-laws.” On February 16th, 1838, he was appointed Secretary of the Geological Society, a position which he retained until February 1st, 1841. During these two years after the voyage he saw much of Sir Charles Lyell, whose teachings had been of the greatest help to him during the voyage, and whose method of appealing to natural causes rather than supernatural cataclysms undoubtedly had a most important influence on the development of Darwin’s mind. This influence he delighted to acknowledge, dedicating to Lyell the second edition of his “Voyage,” “as an acknowledgment that the chief part of whatever scientific merit this ‘Journal’ and the other works of the author may possess has been derived from studying the well-known and admirable ‘Principles of Geology.’” [Sidenote: EARLY WORKS.] At this period he finished his “Journal,” which was published in 1839 as Vol. III. of the “Narrative of the Surveying Voyages of Her Majesty’s Ships _Adventure_ and _Beagle_.” A second edition was published in a separate form in 1845 as the “Journal of Researches into the Natural History and Geology of the Countries visited during the Voyage of H.M.S. _Beagle_ round the World, under the command of Captain Fitz-Roy, R.N.”; and a third edition--but very slightly altered--in 1860, under the title “A Naturalist’s Voyage: Journal of Researches, etc.” This book is generally admitted to deserve above all others the generous description which Darwin gave to Sir Joseph Hooker of Belt’s admirable “Naturalist in Nicaragua”--as “the best of all Natural History journals which have ever been published.”