APPENDIX III 263 



reasoning leads to this decision. Protozoa and Metazoa consist 

 of successive generations of cells ; in the former the cells sep- 

 arate ; in the latter they remain united ; the death of a Protozoon 

 is the annihilation of a cell, but the death of a Metazoon is 

 the dissolution of the union of cells. Such a dissolution is the 

 result of time, that is to say, of the period necessary to the 

 natural duration of life, and we call it, therefore, ' natural 

 death.' Moreover, we know that natural death is brought about 

 by gradual changes in the cells until, at last, certain cells, which 

 are essential to the preservation of the whole, cease their func- 

 tions. Death, therefore, is a consequence of changes which 

 progress slowly through successive generations of cells. These 

 changes cause senescence, the end of which is death. If we wish 

 to know whether death, in the sense of natural death, properly so 

 called, occurs in Protozoa or not, we must first possess some mark 

 or sign, by which we can determine the occurrence or absence of 

 senescence in unicellular organisms. 



"Around this point the whole discussion revolves. Certainly 

 a simpler and more certain conclusion could hardly be drawn 

 than that the death of a Metazoon is not identical, i.e., homol- 

 ogous, with the death of a single cell. Weismann tacitly assumed 

 precisely this homology, and bases his whole argument on it. 

 In all his writings upon this subject, he regards the death of a 

 Protozoon as immediately comparable with the death of a Meta- 

 zoon. If we seek from Weismann for the foundation of this view 

 we shall have only our labour for our pains. Starting from this 

 view Weismann comes to the strictly logical conclusion that the 

 Protozoa are immortal. This is a paradox ! In fact, if one 

 compares death in the two cases, from Weismann's standpoint, 

 then we must assume a difference in the causes of death, and 

 conclude that the cause in the case of the Protozoa is external 

 only, while in the Metazoa it is internal only, for, of course, we 

 may leave out of account the accidental deaths of Metazoa. If 

 we approach the problem from this side, we encounter the 

 following principal question : Does death from inner causes 

 occur in Protozoa ? Weismann gives a negative answer to this 



