_Possible Extinction of a Crystalline Species._—As these crystals melt at 18°, this temperature represents the point of fusion of solid glycerine or the point of solidification of liquid glycerine. But the liquor does not solidify at all if its temperature falls below 18° C., as we well know, for it is at that temperature we use it. Nor does it solidify at zero, nor even at 18° below zero; at 20°, for instance, it merely thickens and becomes pasty. We only know glycerine, then, in a state of superfusion, a fact which chemists have not learned without amazement. Under these conditions, so analogous to the appearance of a living species, to its unlimited propagation and to its extinction, the mineral world offers a quite faithful counterpart to the animal world. The living body illustrates here the history of the brute body and facilitates its exposition. Inversely, the brute body in its turn throws remarkable light on the subject of the living body, and on one of the most serious problems relative to its origin, that of spontaneous generation. _Conclusion._—These facts lead to one conclusion. Until the concourse of propitious circumstances favourable to their spontaneous generation was brought about, crystals were obtained only by filiation. Until the discovery of electro-magnetism, magnets were made only by filiation, by means of the simple or double application of a pre-existing magnet. Before the discovery which fable attributes to Prometheus, every new fire was produced only by means of a spark from a pre-existing fire. We are at the same historical stage as regards the living world, and that is why, up to the present, there has never been formed a single particle of living matter except by filiation, except by the intervention of a pre-existing living organism. BOOK V. SENESCENCE AND DEATH. Chap. I. The different points of view from which death may be regarded.—Chap. II. Constitution of the organisms—Partial death—Collective death.—Chap. III. Physical and chemical characteristics of cellular death—Necrobiosis.— Chap. IV. Apparent perennity of complex individuals.—Chap. V. Immortality of the protozoa and of slightly differentiated cells.