Secondly: it is after all probable that the infusoria are brought to a natural death through their own vital process, for the contradiction between Woodruff’s findings and those of others arises from Woodruff having placed each generation in fresh nutrient fluid. When he refrained from doing so he observed, as did the other investigators, that the generations showed signs of age. He concluded that the animalculae were injured by the products of metabolism which they gave off into the surrounding fluid, and was then able to prove convincingly that only the products of _its own_ metabolism had this effect in bringing about the death of the generation. For in a solution over-saturated with waste products of a distantly related species the very same animalculae throve excellently which when allowed to accumulate in their own nutrient fluid inevitably perished. Thus, left to itself, the infusorium dies a natural death from the imperfect disposal of its own metabolic products: perhaps all higher animals die ultimately from the same inability. At this point the doubt may then occur to us whether any good purpose has been served in looking for the answer to the question as to natural death in the study of the protozoa. The primitive organisation of these forms of life may conceal from us important conditions which are present in them too, but can be recognised only among the higher animals where they have achieved for themselves a morphological expression. If we abandon the morphological point of view for the dynamic, it may be a matter of entire indifference to us whether the natural death of the protozoa can be proved or not. With them the substance later recognised as immortal has not yet separated itself in any way from the part subject to death. The instinctive forces which endeavour to conduct life to death might be active in them too from the beginning and yet their effect might be so obscured by that of the forces tending to preserve life that any direct evidence of their existence becomes hard to establish. We have heard, it is true, that the observations of biologists allow us to assume such death-ward tending inner processes also among the protozoa. But even if the protozoa prove to be immortal in Weismann’s sense, his assertion that death is a late acquisition holds good only of the outward manifestations of death, and does not invalidate any hypothesis as to such processes as impel towards death. Our expectation that biology would entirely put out of court any recognition of the death-instincts has not been fulfilled. It is open to us to occupy ourselves further with this possibility, if we have other reasons for doing so. The striking resemblance between Weismann’s separation of soma and germ-plasm and our distinction between the death and the life-instincts remains unshaken, moreover, and retains its value.