It is difficult to give any confident answer to this question. We cannot reach clearness on this point through our present knowledge of the germ-plasm, because we possess no insight into its structure; we can only draw conclusions as to the processes in the germ-plasm from the observed phenomena of variation and inheritance. But two facts stand in direct antithesis to one another, first, the high power of adaptation possessed by all species, and the undoubted occurrence of unrestricted persistence in a given direction of variation, as seen in artificial selection, and in the disappearance of parts which have ceased to function; and, secondly, the great constancy of old-established species which do indeed always exhibit a certain degree of individual variability, but without showing marked deviations as a frequent occurrence or in all possible directions, as they certainly would if every determinant favoured by a chance increase in the nutritive stream necessarily and irresistibly went on varying further in the same direction. Or can the constancy of such species be maintained solely by means of personal selection, which is continually setting aside all the determinants which rise above the selection-value by eliminating their possessors? I was for long satisfied that this was the true solution of the difficulty, and even now I do not doubt that personal selection does, in point of fact, maintain the constancy of the species at a certain level, but I do not believe that this is sufficient, but rather that it is necessary to recognize an equalizing influence due to germinal selection, and to attribute to this a share in maintaining the constancy of a species which has long been well adapted. I am led to this assumption chiefly by the phenomena of variation in Man, for we find in him a thousand kinds of minute hereditary individual variations, of which not one is likely to attain to selection value. Of course the constant recurrence of reducing divisions prevents any particular id which contains a varying determinant from being inherited through many generations; for so many ids are being continually removed from the genealogical tree by the constant rejection of the half of all ids of every germ-plasm, that only a small part of the ancestral id remains in the grandchild, great-grandchild, and so on. Certainly some of the ids of the ancestors compose the germ-plasm of the descendants, and if all the determinants of one of these ids had begun to vary persistently upwards or downwards in an ancestor, then all the determinants of the relative id in the descendants would possess the variation in an intensified degree; and however slowly the variation advanced it would attain selection-value in some one or other of the descendants, and would thus break the previously stable type of the most perfectly adapted species. The descendant in question would then succumb in the struggle for existence. But as the number of the determinants in the germ-plasm is