The explanation of divergence of species during modification (divergence of character) had not then occurred to him, and he tells us in the “Autobiography”:-- “I can remember the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me; and this was long after I had come to Down. The solution, as I believe, is that the modified offspring of all dominant and increasing forms tend to become adapted to many and highly diversified places in the economy of nature.” A good example of this tendency is seen in the relations of three great vertebrate classes--mammals, birds, and fishes--to the environments for which they are respectively fitted: earth, air, and water. Competition is most severe between forms most nearly alike, and hence some measure of relief from competition is afforded when certain members of each of these classes enter the domain of one of the others. Hence, we observe that although mammals as a whole are terrestrial, a small minority are aërial and aquatic; although birds are aërial, a minority are terrestrial and aquatic; although fishes are aquatic, a minority tend to be, at any rate largely, terrestrial and aërial. Huxley considered it “curious that so much importance should be attached to this supplementary idea. It seems obvious that the theory of the origin of species by natural selection necessarily involves the divergence of the forms selected” (“Obituary,” 1888, reprinted in “Darwiniana,” 1893; see pp. 280, 281). But Darwin showed that divergence might be a great advantage in itself, and would then be directly (and not merely incidentally and indirectly) encouraged and increased by natural selection. [Sidenote: RESOLVE TO PUBLISH.] As soon as the 1844 sketch was finished, Darwin wrote a letter (July 5th) as his “solemn and last request” that his wife would, in the case of his death, devote £400, or if necessary £500, in publishing it, and would take trouble in promoting it. He suggests Lyell as the best editor, then Edward Forbes, then Henslow (“quite the best in many respects”), then Hooker (“would be _very_ good”), then Strickland. After Strickland he had thought of Owen as “very good,” but added, “I presume he would not undertake such a work.” If no editor could be obtained, he requested that the essay should be published as it was--stating that it was not intended for publication in its present form. In August, 1854, he wrote on the back of the letter: “Hooker by far best man to edit my Species volume.”