Among the numerous and contradictory views of recent biologists on the nature of death we find many errors and misunderstandings, due to a lack of clear distinction between the duration of the living matter in general and that of the individual life-form. This is particularly noticeable in the contradictory views which have been elicited by August Weismann's theory (1882) of the immortality of the unicellulars. I have shown in the eleventh chapter of the _Riddle_ that it is untenable. But as the distinguished zoologist has again taken up his theory with energy in his instructive _Lectures on the Theory of the Descent_ (1902), and has added to it erroneous observations on the nature of death, I am obliged to return to the point. Precisely because this interesting work gives most valuable support to the theory of evolution, and maintains Darwin's theory of selection and its consequences with great effect, I feel it is necessary to point out considerable weaknesses and dangerous errors in it. The chief of these is the important theory of the germ-plasm and the consequent opposition to the inheritance of acquired characteristics. Weismann deduces from this a radical distinction between the unicellular and the multicellular organisms. The latter alone are mortal, the former immortal; "between the unicellular and the multicellular lies the introduction of physiological--that is to say, normal--death." We must say, in opposition to this, that the physiological individuals (_bionta_) among the protista are just as limited in their duration as among the histona. But if the chief stress in the question is laid, not on the individuality of the living matter, but on the continuity of the metabolic life-movement through a series of generations, it is just as correct to affirm a partial immortality of the plasm for the multicellulars as for the unicellulars.