Man himself affords a particularly good example in favour of this interpretation in the case of so-called 'identical twins.' It is well known that there are two kinds of twins, those that are not strikingly alike, and often very different, and those that are alike to the extent of being mistakable for one another. Among the latter the resemblance may go so far that the parents find it necessary to mark the children by some outward sign, so that they may not be continually confused. We have now every reason to believe that twins of the former kind are derived from two different ova, and that those of the latter kind arise from a single ovum, which, after fertilization, has divided into two ova. This not infrequently occurs in fishes and other animals, and we can bring it about artificially in a number of species by experimentally separating the two first blastomeres. We have here, then, a case of absolute identity of the germ-plasm in two individuals, for the id-combination of the two ova derived from the same process of fertilization must be exactly the same. That in such a case, notwithstanding the inevitable differences of external influences to which the twins are exposed from intra-uterine life onwards, such a high degree of resemblance should arise is a fact of great theoretical importance. From the basis of the germ-plasm theory we can very well understand it, for, according to the theory, only precisely similar combinations of ids can give rise to identical individuals. But we learn more than this from the occurrence of identical twins. They prove above all that the whole future individual is determined at fertilization, or, to express it theoretically, that the id-composition of the germ-plasm is decisive for the whole ontogeny. It might have been supposed that the combination of ids could change again during development, and that a greater multiplication of some than of others might take place at certain stages of development, or through certain chance external influences. It might have been thought that there was a struggle among the ids in the sense that some of them were suppressed and set aside. All such suppositions break down in face of the fact of identical twins, which teaches us that identical germ-plasm evokes an ontogeny which runs its course as regularly as two chronometers, which are constructed and regulated alike. But when I say that a struggle of the ids, in the sense of a material setting aside of some of them, cannot take place, I by no means intend to maintain that the influence which each individual id exerts on the course of development may not be disproportionate to that exerted by others, and, under some circumstances, very disproportionate indeed. I must refrain from entering into this subject in detail now, but I should like to give at least an indication of what I mean.