482 SUMMARY OF CURRENT RESEARCHES RELATING TO 



appears in the most striking light when we endeavour to realize 

 in these lower forms the idea of death, such as we have been led to 

 consider it from observations of the higher animals. Death in the 

 higher organisms is not the total extinction of life, but only of the 

 individual existence ; but the reproduction of a unicellular organism 

 constitutes at the same time its death. On the other hand, however, by 

 the idea of death in the higher organisms is implied an actual separa- 

 tion of organized substance from the activity of life, in other words, 

 an annihilation of previous life. This element is entirely wanting 

 in the individual death of the Protozoa, that is, in its reproduction ; 

 it goes on living all the same, though in the persons of its progeny. 



If we study the development of certain Protozoa, — the Infusoria,- - 

 we come upon the highly remarkable fact that death does not occur in 

 them, in the sense of annihilation of organic material and from causes 

 inherent in the organism itself. Although these organisms, in the 

 course of their life, are threatened by death under a thousand forms, 

 yet this takes place by " accidents," and thus the few individuals 

 which reproduce the species are to be considered of equal importance 

 with the multitude which perish, for the few reproduce by fission only, 

 and are thus immortal ; while the many which die could have repro- 

 duced their species just as well as the others, if they had had the same 

 favourable opportunity; not one of them necessarily carries in it the 

 seeds of death. 



Whoever wishes to construct a hypothetical representation of the 

 fact that in the higher animals the individual is limited in its dura- 

 tion to a certain time, will find a tolerably simple plan open to him. 

 If we hold it to be allowable to consider the peculiar vital manifesta- 

 tions of the cell, the fundamental element of every form of organiza- 

 tion, as caused by the presence of a substance which acts in a certain 

 sense like a ferment, — necessary to the production of those chemical 

 changes in the cell which result in vital manifestations, but gradually, 

 though perhaps slowly, used up, — then the limited duration of the life 

 of one of the higher animals may be intelligibly represented by 

 assuming that the ovum out of which this organism once originated 

 acquired a certain amount of this ferment-like substance, which is 

 gradually exhausted during life, and with the final exhaustion of 

 which the end of the individual existence coincides. 



It is otherwise with the Protozoa, which reproduce by simple 

 division. These organisms have also this characteristic vital ferment, 

 but they also enjoy the peculiarity of being able to renew it ; hence it 

 is not exhausted in them, and they are not overcome by death in con- 

 sequence of its being used up. 



But the power of forming this vital ferment is shared by the 

 higher organisms as well, but here it is localized, being confined to 

 the generative organs. In the other cells composing the body the 

 material we have been speaking of is gradually and increasingly used 

 up in the course of their active existence; but in the generative 

 regions, whose cells maintain their primitive character longest, fresh 

 vital ferment is accumulated for their posterity. Certain appearances 

 occur which, perhaps, justify us in forming an approximate idea as 



