It is to be observed that this idea of potential immortality is not the exclusive property of Weismann. We find it clearly indicated by Ehrenberg. And, moreover, as Bütschli remarks, it is so natural that it ought to occur of itself to the mind of every tolerably thoughtful observer that has devoted his time to the study of the biology of these minute creatures.[12] [12] _Gedanken über Leben und Tod_ (_Zoologische Anzeige_, Vol. v, 1882), cited by M. Maupas in _Multiplication des Infusoires Ciliés—Arch. de Zool. Experimen._, No. 2, 1888. Weismann founded his theory in part upon metaphysical, or at least theoretical, considerations, which we deem it useless to discuss at this point. But it is also supported by observed facts, and these facts it will be profitable to recapitulate from the very onset. The idea of the immortality of Infusoria occurs naturally to the mind when one examines with care what happens when an Infusorian reproduces. We know that the reproduction consists in a bipartition of the body of the animal, and that consequently the parent does not die but lives in the two products of its bipartition. In subsequent multiplications the same phenomenon is always observed to occur, so that the entire substance of the parent is found preserved and living in the individuals to which it gives birth. This process Weismann expressed by the emphatic statement: In multiplication by division there are no corpses. It is wholly otherwise with the metazoans, and the reason of this fundamental difference is easily explained by the comparison of the organisation of the body of a metazoan with that of a protozoan. Whereas the protozoan is represented by a single cell that comprehends all the vital functions, the functions of reproduction as well as those of nutrition and relation, the metazoan, on the other hand, is composed of an aggregation, of a colony of distinct cells, among which a division of labor has been effected varying in complexity with the height that the animal has attained in the classificatory scale. It results from this division of labor that in the metazoan certain cells only—those namely which are called the sexual cells—are entrusted with the office of the conservation of the species, while the various other cells are more especially adapted to the conservation of the individual. When a metazoan reproduces, the sexual cells alone enter into activity, and after having suffered various modifications, the principal one of which is fecundation, the sexual cells become the seat of numerous segmentations that go to constitute a new animal distinct from the one that gave it birth. The moment the parent individual ceases to be blended with the individual it produces, it can perish without imperilling the conservation of the species, and thus it is that death appears in the animal kingdom as a logical consequence of division of labor.