We also know that Weismann, in developing these interesting facts, was led with many other naturalists to establish the doctrine that every metazoan may be considered as made up of two entirely distinct groups of cells: 1) of somatic cells, which represent the individual, and which are invested with the care of its nourishment, its sense-mechanism, its movements, and all the functions that have to do with individual life; and 2) of sexual cells, charged with the office of the maintenance of the species in time. Whereas the somatic cells are destined to perish, the sexual cells on the contrary, multiplying by division after the mode of the reproduction of micro-organisms, represent the protozoan type, which is immortal; and, by the intermediary agency of the fecundated ovum, the sexual cells pass from generation to generation, thus forming a material bond between successive generations. Though we have to succumb to death, there is at least a portion of us that ought not to die, from the fact that it is transmittible to our descendants. Naegeli expressed this idea in a felicitous form, when he compared the species to a creeping branch that sent out at successive points annual buds. The buds, which die, are the individuals—that is the somatic group; while the branch that survives after the death of the buds, and which represents the species, is the system of sexual cells. Weismann, finally, has described the same phenomenon by the expression 'continuity of the germinative plasm.' A great many discussions have arisen with regard to this germinative plasm; for everything touches upon this domain, and Weismann has conceived a theory that endeavors to explain not only the phenomenon of fecundation, but also that of heredity. I cannot mention here the numerous works upon this subject, and refer the inquisitive reader for a knowledge of the same to a series of lectures by Professor Balbiani that I have epitomized in the _Revue Philosophique_ for December 1889. The theory of the potential immortality of the Infusoria has recently been attacked by M. Maupas, whose observations tend to show that natural death, caused by senescence, does obtain among the Infusoria, and that it is comparable in many points of view to the natural death of the metazoans. The researches of M. Maupas upon the multiplication of ciliate Infusoria are of a relatively recent date, having appeared in 1888 in Vol. VI. of the "Archives de Zoologie Expérimentale."