It is still less possible to do any justice in a few words to Weismann's theory. Into its various modifications, as it has grown from year to year, we have no time to enter. And we must confine ourselves to his views of variation and heredity. In studying protozoa we noticed that they reproduced by fission, each adult individual dividing into two young ones. There is therefore no old parent left to die. Natural death does not occur here, only death by violence or unfavorable conditions. The protozoa are immortal, not in the sense of the endless persistence of the individual, but of the absence of death. Heredity is here easily comprehensible, for one-half, or less frequently a smaller fraction, of the substance of the parent goes to form the new individual. There is direct continuity of substance from generation to generation. But in volvox a change has taken place. The fertilized egg-cell, formed by the union of egg and spermatozoon, is a single cell, like the individual resulting from the conjugation or fusion of two protozoa. But in the many-celled individual, which develops out of the fertilized egg, there are two kinds of cells. 1. There are other egg-cells, like the first, each one of which can, under favorable conditions, develop into a multicellular individual like the parent. And the germ-cells (eggs and spermatozoa) of volvox are immortal like the protozoa. But, 2, there are nutritive, somatic cells, which nourish and transport the germ-cells, and after their discharge die. These somatic cells, being mortal, differ altogether from the germ-cells and the protozoa. The protoplasm must differ in chemical, or molecular, or other structure in the two cases, and we distinguish the germ-plasm of the germ-cells, resembling in certain respects Nägeli's idioplasm, from somatoplasm, which performs most of the functions of the cell. The somatoplasm arises from, and hence must be regarded as a modification of, the germ-plasm. The germ-plasm can increase indefinitely in the lapse of generations, increase of the somatoplasm is limited. When a new individual develops, a certain portion of the germ-plasm of the egg is set aside and remains unchanged in structure. This, increasing in quantity, forms the reproductive elements for the next generation. The germ-plasm, which does not form the whole of each reproductive element, but only a part of the nucleus, is thus an exceedingly stable substance. And there is a just as real continuity of germ-plasm through successive generations of volvox, or of any higher plants or animals, as in successive generations of protozoa.