If we do so, we may be astonished to find how little agreement exists among biologists on the question of natural death, that indeed the very conception of death altogether eludes them. The fact of a certain average length of life, at least among the higher animals, is of course an argument for death from inner causes, but the circumstance that certain large animals and giant trees reach a very great age, one not to be computed up to now, once more removes this impression. According to the grandiose conception of W. Fliess all the vital phenomena—and certainly also death—are linked with the accomplishment of certain periods of time, among which there finds expression the dependence of two living substances, one male and one female, upon the solar year. But observations of how easily and extensively the influences of external forces can alter vital manifestations, especially in the plant world, as to their occurrence in time, can hasten or retard them, militate against the rigidity of the formulae laid down by Fliess and leaves at least doubtful the universality of the laws he sought to establish. The treatment of these themes, death and the duration of life among organisms, in the works of A. Weismann[19] possesses the greatest interest for us. This investigator originated the distinction of living substance into a mortal and an immortal half; the mortal is the body in the narrower sense, the soma, which alone is subject to natural death; while the germ cells are potentially immortal, in so far as they are capable under certain favourable conditions of developing into a new individual, or expressed otherwise, of surrounding themselves with a new soma.[20] What here arrests our attention is the unexpected analogy with our conception developed along so different a line of thought. Weismann, who is considering living substance morphologically, recognises in it a constituent which is the prey of death, the soma, the body viewed apart from sex or heredity elements, and, on the other hand, an immortal part, the germ-plasm, which serves the purpose of preservation of the species, of propagation. We have fixed our attention not on the living matter, but on the forces active in it, and have been led to distinguish two kinds of instincts: those the purpose of which is to guide life towards death, and the others, the sexual instincts, which perpetually strive for, and bring about, the renewal of life. This sounds like a dynamic corollary to Weismann’s morphological theory.