8 The American Naturalist. [January, 



sess some mark or sign, by which we can determine the occur- 

 rence 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 tacitty as- 

 sumed 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 Metazoon. If we seek from Weismann for the 

 foundation of this view we shall have only our labor 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 ap- 

 proach 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 ques- 

 tion, with his assertion that unicellular organisms are immor- 

 tal. The assertion remains, but the proof of the assertion is 

 lacking. In order to justify the assertion, it must be demon- 

 strated that there does not occur in Protozoa a true senescence, 

 showing itself gradually through successive generations of 

 cells. Has Weismann furnished this demonstration ? Cer- 

 tainly not. He has, strictly speaking, not discussed the sub- 

 ject. It is clear that we must first determine whether natural 

 death from senesence occurs in Protozoa or not, before we can 

 pass to a scientific discussion of the asserted immortality of 

 unicellular beings. The problem cannot be otherwise appre- 

 hended. Weismann has not thus conceived it, therefore the 

 judgment stands against him : he misses the real }>roblem. 



Senesence has been hitherto little investigated ; for many 

 years I have been studying it experimentally and have tried 



