The immortality of the unicellulars, on which Weismann has laid so much stress, can only be sustained for a small part of the protists even in his own sense--namely, for those which simply propagate by cleavage, the chromacea and bacteria among the monera (chapter ix.), the diatomes and paulotomes among the protophyta, and a part of the infusoria and rhizopods among the protozoa. Strictly speaking, the individual life is destroyed when a cell splits into two daughter-cells. One might reply with Weismann that in this case the dividing unicellular organism lives on as a whole in its offspring, and that we have no corpse, no dead remains of the living matter, left behind. But that is not true of the majority of the protozoa. In the highly developed ciliata the chief nucleus is lost, and there must be from time to time a conjugation of two cells and a mutual fertilization of their secondary nuclei, before there can be any further multiplication by simple cleavage. However, in most of the sporozoa and rhizopoda, which generally propagate by spore formation, only one portion of the unicellular organism is used for this; the other portion dies, and forms a "corpse." In the large rhizopods (thalamophora and radiolaria) the spore-forming inner part, which lives on in the offspring, is smaller than the decaying outer portion, which becomes the corpse. Weismann's view of the secondary "introduction of physiological death in the multicellulars" is just as untenable as his theory of the immortality of the unicellulars. According to this opinion, the death of the histona--both the metaphyta and metazoa--is a purposive outcome of adaptation, only introduced by selection when the multicellular organism has reached a certain stage of complexity of structure, which is incompatible with its original immortality. Natural selection would thus kill the immortal and preserve only the mortal; it would interfere with the multiplication of the immortals in the bloom of their years, and only use the mortal for rearing posterity. The curious conclusions which Weismann reached in developing this theory of death, and the striking contradictions to his own theory of the germ-plasm which he fell into, have been pointed out by Kassowitz in the forty-ninth chapter of his _General Biology_. In my opinion, this paradoxical theory of death has no more basis than the germ-plasm theory he has ingeniously connected with it. We may admire the subtlety and depth of the speculations with which Weismann has worked out his elaborate molecular theory. But the nearer we get to its foundations the less solid we find them. Moreover, not one of the many supporters of the theory of germ-plasm has been able to make profitable use of it in the twenty years since it was first published. On the other hand, it has had an evil influence in so far as it denied the inheriting of acquired characters, which I hold, with Lamarck and Darwin, to be one of the