We are well aware, especially from observation on ourselves, that is, on Man, that the children of a pair often resemble one another but are never alike, and that one child frequently resembles one parent, another the other, while a third may exhibit a mingling of both parents. How does this come about? Since the germinal substance of both parents is derived from that of the ovum, from which they themselves have arisen--and must therefore be the same in all the germ-cells to which they give rise--new determinants cannot be added, and old ones cannot be dropped out, and variation of the determinants, the possibility of which is granted, would still not directly bring about the familiar mingling of resemblances to the two parents, but would at most give rise to something new and strange. Here the theory helps to elucidate matters. We found ourselves obliged to assume that the germ-plasm is composed of ids, that is, of equivalent portions of germ-plasm, each of which contains all the kinds of determinants appertaining to the building up of an individual, but each of these kinds in a particular individual form. I have already called these ids 'ancestral plasms,' and the term is appropriate, in so far that in every fertilization an equal number of ids from the father and from the mother are united in the ovum, so that the child is built up of the ids of his two nearest 'ancestors.' But as the ids of the parents are derived from those of the grandparents, and these again from those of the great-grandparents, the ids are in truth the idioplasm of the ancestors. The expression, however, has been very frequently misunderstood, as if it were intended to mean that the ids retained _unchanged_ for all time the character of their respective ancestors, and I have even been credited with supposing that our own ids still consist of the determinant-complexes of our fish-like or even Amœba-like ancestors. But in reality no id exactly or completely corresponds to the type, that is, to the whole being of any one of the ancestors in whose germ-plasm it was formerly contained, for each of the ancestors had many ids in his germ-plasm, and his entire constitution was not determined by any one of these alone, but by the co-operation of them all. The individual arising from a germ-cell must necessarily be the result of all the ids which make up his germ-plasm, but undoubtedly the share taken by some of them may be much stronger than that taken by others. It is also clear that, if we leave out of account any possible variation on the part of the ids, each of them belongs, not to one ancestor only, but to a whole series of ancestors, and must have taken part in their development, so that it is not the idioplasm of any particular ancestor, but only ancestral plasm in the general sense. In this sense we may quite well retain the designation, 'ancestral plasm,' for the id.