existence among organisms both against each others’ competition and against the general conditions of life[57]—given these simple data, the secret springs of evolution are laid bare, and the vast complexity of natural forms upon the globe is adequately accounted for without calling in the agency of special creations. But variations are the starting-point in the process: Natural Selection can originate nothing—it can only act on what is presented to it by some quite different force. The relative parts played by the various agencies at work are, with characteristic moderation of statement, thus described by Darwin:— “On the whole I think we may conclude that habit, use, and disuse, have, in some cases, played a considerable part in the modification of the constitution, and in the structure of various organs; but that the effects of use and disuse have often been largely combined with, and sometimes overmastered by the natural selection of innate variations.”[58] To explain evolution, then, we must first explain _the occurrence of appropriate variations_, strong enough and widespread enough to maintain themselves against the constant reducing influence of promiscuous intercrossing, and they must be variations _capable of being transmitted by inheritance_. This, we now see, is the true field of the inquiry. The new factors introduced by Darwin into the process of evolution—Natural Selection and Innate Variations—were destined in our day to have the whole weight of the argument for evolution suddenly thrown upon them. The inheritability of variations acquired by the individual through use and disuse when subjected to fresh investigation by the younger school of biologists has turned out to be open to the gravest doubts, both theoretically, on account of the great difficulty of reconciling it with what has now been ascertained of the nature of the reproductive mechanism in plants and animals, and also on the score of a closer consideration of the facts commonly adduced as evidence for the law. To take these points separately: The reproductive cells in every living creature are now believed to be formed directly from the reproductive cells of its parents. They are not a product of the organism in which they find themselves. They are nourished by its blood, and are therefore liable to be affected by anything which produces a broad general effect on the constitution of the being in whom they are lodged, but it is difficult to see how special modifications of individual parts of that being could affect them so as to influence the determinants in the direction of reproducing that modification. How, for example, could the habit of grubbing for roots in an animal of the pig tribe so affect its reproductive cells as to ensure the birth of an offspring with callosities on their snouts? The physiological mechanism by which such a result could be produced seems hardly conceivable—at any rate no one has yet offered a plausible