DEATH 



subtlety and depth of the speculations with which 

 Weismann has worked out his elaborate molecular 

 theory. But the nearer we get to its foundations the less 

 solid we find them. Moreover, not one of the many 

 supporters of the theory of germ-plasm has been able 

 to make profitable use of it in the twenty years since it 

 was first published. On the other hand, it has had an 

 evil influence in so far as it denied the inheriting of 

 acquired characters, which I hold, with Lamarck and 

 Darwin, to be one of the soundest and most indis- 

 pensable supports of the theory of descent. 



In discussing the question of the real causes of death, 

 we confine our attention to normal or physiological death 

 without considering the innumerable causes of accidental 

 or pathological death, by illness, parasites, mishaps, etc. 

 Normal death takes place in all oirganisms when the 

 limit of the hereditary term of life is reached. This 

 limit varies enormously in different classes of organisms. 

 Many of the unicellular protophyta and protozoa live 

 only a few hours, others several months or years; many 

 one-year plants and lower animals live only a summer in 

 our temperate climate, and only a few weeks or months 

 in the arctic circle or on the snow-covered Alps. On the 

 other hand, the larger vertebrates are not uncommonly a 

 hundred years old, and many trees live for a thousand 

 years. The normal span of life has been determined in 

 all species in the course of their evolution by adaptation 

 to special conditions, and has then been transmitted to 

 offspring by heredity. In the latter, however, it is often 

 subject to considerable modifications. 



The organism has been compared, on the modem 

 "machine theory" of life, to an artificially constructed 

 mechanism, or an apparatus in which the human intelli- 

 gence has put together various parts for the attainment 

 of a certain end. This comparison is inapplicable to the 

 lowest organisms, the monera, which are devoid of such 



lOI 



