Let us recall what we have already established in regard to the evolutionary power of natural selection, namely, that the changes which it controls can never transcend the range of their utility, and it will be clear to us that, of the many ids which make up the germ-plasm of the species, only so many will be modified as are necessary to evoke the character which has varied. Just as the protective resemblance of an insect to a leaf may be raised to a very high pitch, but can never become perfect, because an imperfect resemblance is already sufficient to deceive the persecutor, and the selective process comes to a standstill because individuals which possessed a still greater resemblance to a leaf would be no better protected from destruction than the others, so in the modification of a species the whole of the ids need not at once be modified, if a majority is sufficient to stamp the great majority of individuals with the desired variation. But it may happen that, at the reduction of ids during the development of the germ-cells, an id-combination with wholly or almost wholly unchanged ids may come together in one germ-cell, and if another sperm-cell of this kind meets with an egg-cell similarly constituted, an individual of the old species must arise. But this must--on our assumption--be at a disadvantage as compared with the transformed individuals in the struggle for existence, and will perish in it, and therefore the number of unmodified ids in the germ-plasm of the species will gradually diminish. It is obvious, however, that this will take place very slowly, as we may conclude from the phenomena of reversion, of which I shall have to speak later on. But what is true of the ids is true also of their constituent parts, the determinants, and that--if I mistake not--is fundamental in the interpretation of the alternation of hereditary succession in the parts of the child.