Weismann's explanation of heredity is at first sight relatively simple. In reply to the question, "Why is the offspring like the parent?" he says, "Because it is composed of some of the same stuff." In other words, there has been unbroken germinal continuity between generations. His idea of germinal continuity, _i.e._, unbroken continuity, through all time, of the germinal substance, is a conception of very great extent, and now underlies all discussion of heredity. In order to comprehend it, we must first distinguish between the germ-cells and the body-cells. Weismann regards the body, composed of its many cells, as a derivative that becomes simply a vehicle for the germ-cells. Owen's distinction between germ-cells and body-cells, made in 1849, was not of much importance, but in the theory of Weismann it is of vital significance. The germ-cells are the particular ones which carry forward from generation to generation the life of the individual. The body-cells are not inherited directly, but in the transmission of life the germ-cells pass to the succeeding generation, and they in turn have been inherited from the previous generation, and, therefore, we have the phenomenon of an unbroken connection with all previous generations. When the full significance of this conception comes to us, we see why the germ-cells have an inherited organization of remarkable complexity. This germinal substance embodies all the past history of the living, impressionable protoplasm, which has had an unbroken series of generations. During all time it has been subjected to the molding influence of external circumstances to which it has responded, so that the summation of its experiences becomes in some way embedded within its material substance. Thus we have the germinal elements possessing an inherited organization made up of all the previous experiences of the protoplasm, some of which naturally are much more dominant than the others. We have seen that this idea was not first expressed by Weismann; it was a modification of the views of Nussbaum and Hertwig. While it was not his individually, his conclusions were apparently reached independently. This idea was in the intellectual atmosphere of the times. Several investigators reached their conclusions independently, although there is great similarity between them. Although the credit for the first formulation of the law of germinal continuity does not belong to Weismann, that of the greatest elaboration of it does. This doctrine of germinal continuity is now so firmly embedded in biological ideas of inheritance and the evolution of animal life that we may say it has become the corner-stone of modern biology. The conclusion reached--that the hereditary substance is the germ-plasm--is merely preliminary; the question remains, Is the germ-plasm homogeneous and endowed equally in all parts with a mixture of hereditary qualities? This leads to the second step.