342 GENERAL PHYSIOLOGY 



causes. Hence, according to Weismann's idea, unicellular organisms 

 are immortal. Weismann, therefore, disputes the view that death 

 is a phenomenon grounded in the nature of living substance, and 

 does not believe that it depends upon '' purely internal causes 

 inherent in the nature of life itself." He holds death, rather, to 

 be a phenomenon of adaptation which has been evolved in the 

 course of the organic development of the earth as advantageous, 

 and he represents its appearance in the organic series somewhat 

 as follows : In the unicellular Protista all the functions of the 

 body including that of reproduction are localised in a single cell. 

 If, therefore, natural death were a necessity for the unicellular 

 organism, reproduction would terminate with death ; and, since 

 with the equality of the parts resulting from division the same 

 holds good for all, after a short time the species in question would 

 become extinct. Hence in unicellular forms death is impossible, 

 Weismann maintains, because otherwise the species would become 

 extinct. In multicellular organisms, on the other hand, the higher 

 we go in the series, the more a contrast develops between the 

 sexual cells, which serve for reproduction only and hence for the 

 maintenance of the species, and the cells of the rest of the body, 

 which in the higher animals have completely lost the power of 

 reproducing the species. Here, therefore, there is the possibility 

 of death without the maintenance of the species thereby being 

 endangered ; for, if only one reproductive cell really reproduces, if 

 only one egg develops, all the rest of the body can die without the 

 species becoming extinct. Since now, as Weismann says, " the 

 unlimited duration of the individual would be a luxury without 

 any advantage," according to the well-known principles of selection 

 immortality has been lost as disadvantageous and death has been 

 evolved. " In unicellular animals it was impossible to establish 

 normal death because the individual and the reproductive cell 

 were one and the same ; in multicellular organisms, however, 

 somatic and reproductive cells were separate, death became possible, 

 and we see that it was established." 



It cannot be denied that these deductions of Weismann sound 

 very plausible ; nevertheless, they are not invulnerable, and have 

 already called forth much active contradiction. 



Especially has the claim always been contested that unicellular 

 organisms should be considered immortal for the single reason 

 that their body never becomes a corpse. In defining the concep- 

 tion of death, emphasis has been laid by Weismann's opponents 

 largely upon the cessation of the individual^ life, and it has been 

 said : If the unicellular organism divides into halves, its individual 

 existence is therewith ended ; but where the individual existence 

 ceases, the term immortality cannot be used, since in reality the 

 individual has perished; death and reproduction here coincide. 

 It is evident that here there is simply a contest over ideas, which 



