“But you must recollect that when I say I think it is either Mr. Darwin’s hypothesis or nothing; that either we must take his view, or look upon the whole of organic nature as an enigma, the meaning of which is wholly hidden from us; you must understand that I mean that I accept it provisionally, in exactly the same way as I accept any other hypothesis.” He concludes the lectures and the volume in which they are now reproduced by the following eloquent testimony to the unique value of the “Origin of Species”:-- “I believe that if you strip it of its theoretical part it still remains one of the greatest encyclopædias of biological doctrine that any one man ever brought forth, and I believe that, if you take it as the embodiment of an hypothesis, it is destined to be the guide of biological and psychological speculation for the next three or four generations.” The next essay from which I quote was written in 1871. At the beginning of “Mr. Darwin’s Critics” (“Darwiniana,” p. 120) he uses words which, if they stood alone, might be interpreted as an indication of a stronger conviction. “Whatever may be thought or said about Mr. Darwin’s doctrines, or the manner in which he has propounded them, this much is certain, that, in a dozen years, the ‘Origin of Species’ has worked as complete a revolution in biological science as the ‘Principia’ did in astronomy--and it has done so, because, in the words of Helmholtz, it contains an ‘essentially new creative thought.’” This last quotation, and the following one, from “Evolution in Biology,” written in 1878, are, I think, among the strongest utterances in favour of natural selection to be found in the Collected Essays. At the conclusion of the above-named essay (_l. c._, p. 223) he states that it was clearly seen that-- “if the explanation would apply to species, it would not only solve the problem of their evolution, but that it would account for the facts of teleology, as well as for those of morphology;...” “How far ‘natural selection’ suffices for the production of species remains to be seen. Few can doubt that, if not the whole cause, it is a very important factor in that operation; and that it must play a great part in the sorting out of varieties into those which are transitory and those which are permanent.”