In regard to the life-transmitting mechanism the animals are divided somewhat differently. On one side budding and division, or scissiparity, is prolonged rather far into the metazoaire series concurrent with sexual reproduction; on the other hand there are, among protozoaires, phenomena of coupling, unions of cellules which resemble veritable fecundation and perform its rôle; without the nuclear regeneration which is the aim and consequence, neither segmentation nor budding can take place, at least not indefinitely. In sum, the reproduction of beings is always sexual; only in the one case, the protozoaires, it is produced by non-differentiated elements; and in the other, the metazoaires, by differentiated elements, a male and a female. If one clips off bits of a sponge, a hydra, one obtains as many new individuals, which when they have grown one may again divide, and so on repeatedly, but not indefinitely. At a variable instant, after a certain number of generations by fragmentation, senescence appears among the so produced individuals; the clipped morsels remain inert. Thus this sort of artificial virgin birth has a limit, as has normal parthenogenesis, and in order that the individuals may regain their parthenogenetic force one must give them time to regenerate their cellules by the coupling which fecundates them. Fecundation is in all cases, doubtless, merely a rejuvenation, thus considered it is uniform not only throughout the animal series, but throughout the vegetable. One ought to experiment in slip-cutting, and discover at what point the slip cut from a slip begins to diminish in vitality. Coupling and fecundation have the same result: it is necessary that cellules A unite with cellules B (macro-nucleus and micro-nucleus among protozoaires; ovule and spermatozoid among metazoaires), in order that the organism may usefully exteriorize a part of its substance. When the too complex organism has lost the primitive faculty of segmentation, it makes use, directly, to reproduce itself, of certain cellules differentiated for that purpose: it is these cellules united into a whole, which reintegrate and give birth to a double of the generating individual or individuals. From the top to the bottom of the sexual scale the new being springs invariably from a duality. The multiplication takes place only in space. In time the product is a contraction, two giving one.