I find that the late Mr. G. H. Lewes has insisted on this objection in his 'Physical Basis of Mind.' I observe, also, that in the very passage in which he does so, Mr. Lewes appears to have been misled by Mr. Darwin's use of that dangerous word "means," and, at the same time, by his frequent treatment of natural selection as though it were an active cause; so that Mr. Lewes supposes Mr. Darwin to have fallen into the very error of which, as I have above shown, he is evidently struggling to keep clear--namely, that of maintaining natural selection to be a "cause" of variation. Mr. Lewes then continues:-- "He [Mr. Darwin] separates Natural Selection from all the primary causes of variation either internal or external--either as results of the laws of growth, of the correlations of variation, of use and disuse, &c., and limits it to the slow accumulation of such variations as are profitable in the struggle with competitors. And for his purpose this separation is necessary. But biological philosophy must, I think, regard the distinction as artificial, _referring only to one of the great factors in the production of species_."[355] The fact that one in a brood or litter is born fitter for the conditions of its existence than its brothers and sisters, and, again, the causes that have led to this one's having been born fitter--which last is what the older evolutionists justly dwelt upon as the most interesting consideration in connection with the whole subject--are more noteworthy factors of modification than the factor that an animal, if born fitter for its conditions, will commonly survive longer in the struggle for existence. If the first of these can be explained in such a manner as to be accepted as true, or highly probable, we have a substantial gain to our knowledge. The second is little--if at all--better than a truism. Granted, if it were not generally the case that those forms are most likely to survive which are best fitted for the conditions of their existence, no adaptation of form to conditions of existence could ever have come about. "The survival of the fittest" therefore, or, perhaps better, "the fertility of the fittest," is thus a _sine qua non_ for modification. But, as we have just insisted, this does not render "the fertility of the fittest" an especial "means of modification," rather than any other _sine qua non_ for modification. But, to look at the matter in another light. Mr. Darwin maintains natural selection to be "the most important but not the exclusive means of modification." For "natural selection" substitute the words "survival of the fittest," which we may do with Mr. Darwin's own consent abundantly given. To the words "survival of the fittest" add what is elided, but what is, nevertheless, unquestionably as much implied as though it were said openly whenever these words are used, and without which "fittest" has no force--I mean, "for the conditions of their existence."