I can only remember a couple of sentences in the later editions of the “Origin of Species” in which Mr. Darwin directly admits a change of opinion as regards the main causes of organic modification. How shuffling the first of these is I have already shown in “Life and Habit,” p. 260, and in “Evolution, Old and New,” p. 359; I need not, therefore, say more here, especially as there has been no rejoinder to what I then said. Curiously enough the sentence does not bear out Mr. Spencer’s contention that Mr. Darwin in his later years leaned more decidedly towards functionally produced modifications, for it runs: { 161a}—“In the earlier editions of this work I underrated, as now seems probable, the frequency and importance of modifications due,” not, as Mr. Spencer would have us believe, to use and disuse, but “to spontaneous variability,” by which can only be intended, “to variations in no way connected with use and disuse,” as not being assignable to any known cause of general application, and referable as far as we are concerned to accident only; so that he gives the natural survival of the luckiest, which is indeed his distinctive feature, if it deserve to be called a feature at all, greater prominence than ever. Nevertheless there is no change in his concluding paragraph, which still remains an embodiment of the views of Erasmus Darwin and Lamarck.
The other passage is on p. 421 of the edition of 1876. It stands:—“I have now recapitulated the facts and considerations which have thoroughly” (why “thoroughly”?) “convinced me that species have been modified during a long course of descent. This has been effected chiefly through the natural selection of numerous, successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner, that is, in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously. It appears that I formerly underrated the frequency and value of these latter forms of variation as leading to permanent modifications of structure independently of natural selection.”