Mr. Herbert Spencer affirms that “successive changes of conditions would produce divergent varieties or species” of the organisms subject to them, apart from the influence of “natural selection,” which, in the absence of such successive changes of conditions, would effect “comparatively little.”[383] It is to the latter especially Mr. Spencer traces the gradual evolution of nature, on the process of which he has thrown so much light. Thus, when treating elsewhere of that evolution, he says, “While we are not called on to suppose that there exists in organisms any primordial impulse which makes them continually unfold into more heterogeneous forms; we see that a liability to be unfolded arises from the actions and reactions between organisms and their fluctuating environments. And we see that the existence of such a cause of development pre-supposes the non-occurrence of development where this fluctuation of actions and reactions does not come into play.”[384] It is evident that this theory, like that of Mr. Darwin, supposes the occurrence of slight structural changes which, in the absence of knowledge as to their exciting causes, may be described as “spontaneous,” and the perpetuation of which is the establishment of new forms or species. But among domestic animals, and by analogy we may assume, therefore, among wild animals, variation in the way supposed is not the only mode by which the physical structure may be modified. Various instances of sudden change have been collected which are very difficult to deal with, and they have led Mr. Huxley to remark that Mr. Darwin’s position “might have been even stronger than it is if he had not embarrassed himself with the aphorism ‘_natura non facit saltum_,’ which turns up so often in his pages.” Mr. Huxley adds “that nature does make jumps now and then, and a recognition of the fact is of no small importance in disposing of many minor objections to the doctrine of transmutation.”[385] Minor objections may certainly be thus removed, but only by introducing one of much greater moment. If, as Mr. Spencer says, “natural selection is capable of _producing_ fitness between organisms and their circumstances,”[386] it must be by the perpetuation of slight changes, and there does not, indeed, appear to be any room in the hypothesis of natural selection for the saltatory movements which it is so necessary to explain.