whole ontogeny would totter, in fact it would be much as if a man who wished to remove the breadth of a window from a house standing on pillars were to begin by taking away the foundation pillar. It is, of course, to be understood that these processes go on so exceedingly slowly that personal selection takes a share in them, at least at the beginning. Later on, the further degeneration of a useless organ or rudiment has no effect on the individual's power of life, and therefore depends solely upon the struggle of the parts within the germ-plasm (germinal selection). If we could see the determinants, and recognize directly their arrangement in the germ-plasm and their importance in ontogeny, we should doubtless understand many of the phenomena of ontogeny and their relation to phylogeny which must otherwise remain a riddle, or demand accessory hypotheses for their interpretation. Several years ago Emery rightly pointed out that the phenomena of the variation of homologous parts might be inferred by reasoning from the germ-plasm theory. If one hand has six fingers instead of five, it not infrequently happens that the other also exhibits a superfluity of fingers, and sometimes the foot does so too. The phyletic modification of the limbs in the Ungulates has taken place with striking uniformity in the fore and hind extremities; no animal has ever been one-hoofed in front and two-hoofed behind. Although I might suggest that this primarily depends on adaptation to different conditions of the ground, and that the Artiodactyls were evolved in relation to the soft marshy soil of the forest, and the Perissodactyls for the steppes, it cannot be denied that germinal conditions may have co-operated in bringing about this uniformity of the direction of variation, especially as the whole structure of the fore- and hind-limbs exhibits such marked similarity. Emery is inclined to refer this to 'germ-plasmic correlations,' and we have assumed from the very first that the different determinants and groups of determinants do indeed stand in definite and close relations to one another. But it seems to me premature to say anything more precise and definite than that in the meantime. I should like, however, to say that determinants or groups of determinants which had in old ancestral germ-plasms to give rise to a series of quite similar structures by multiplication during the ontogeny, and therefore only needed to be present _singly_ in the germ-plasm, would, in later descendants, have to shift their multiplication back into the germ-plasm itself, if necessity required that the homologous parts which they controlled should become _different_ from each other. Then the previously single group of determinants in the germ-plasm would have to become multiple. But as new determinants can only arise from those which already exist, these new ones must have had their place beside the old, and would therefore probably be exposed to