The world of living beings is a gigantic tree and from its trunk shoot forth the branches of classes, orders, species. On the branches leaves grow, which are the individuals; but each one of these small individuals generates within itself many cells, true organisms within greater ones. The world of living beings is but a great laboratory of prolific, incessant generation. Cells generate cells; organs, organs; species, species. An intimate brotherhood makes us members of one great organism--the placenta of living beings; and among ourselves we exchange the same matter which each of us in turn contributes to the work of apparent destruction, called _nutrition_, and to that of reproduction, designated as _generation_. To feed themselves and to generate, living beings are continually exchanging with each other a part of their own matter which, passing from one organism to another, seems to acquire new energy and new life. On the one hand, seaweeds live on mushrooms, carnivorous animals devour herbivorous, herbivorous feed on herbs, and man, the highest branch of the tree of living beings, partakes of all. On the other hand, males and females in continuous succession interchange part of their matter, remoulding their primitive forms. The most elementary form of life is not, however, the cell, since at a lower stage we find the _protoplasm_, the true _primum vivens_ which, by scission, generates the individual; and, by nourishing itself, nobody can tell what mysterious genesis of atoms it induces within its own most simple organism. The protoplasm cannot live without a continual exchange of matter, so that the live molecules of yesterday are dead today; and those which are alive today will be dead tomorrow; therefore nutrition also, in the last analysis, is an intimate and very mysterious generation. Evanescence of forms is one of the most essential characteristics of living beings, and we give the name of death to the falling of every leaf from the tree of life. Man, also, drops some of these leaves every day--hair, epithelia, cells, which often produce a secretive substance and fall with it. Before dying, a part of the preëxisting form remains to re-animate the dead form and follows in its turn the parabolical cycle through which the mother form has passed. This is the most general principle and includes all possible kinds of generation, from that of scission to the highest form of sexual genesis. One would say that the life of an individual is only a moment of the great life of the species, of the classes, of the kingdoms of living beings; it is a spark which shoots off intermittently, passing from one organism to another.