weismann's theory of heredity. 437 



the species should always be recuperated by fresh, youug, and well- 

 formed represcDtatives. Consequently also, natural selection would 

 speedily see to it that all sexually-propagating species should become 

 deprived of the aboriginal endowment of immortality, with the result 

 that death is now a universal destiny among all the individuals of such 

 species, that is to say, among all the metazoa and metaphyta. I^Iever- 

 theless, it is to be remembered that this destiny extends only to the 

 parts of the individual other than the contents of those specialized cells 

 which constitute the reproductive elements, for although in each in- 

 dividual metazoon or metaphyton an innumerable number of these 

 specialized cells are destined to perish during the life and with the 

 death of the organism to which they belong, this is only due to the 

 accident, so to speak, of their contents not having met with their com- 

 plements in the opposite sex; it does not belong to their essential 

 nature that tbey should perish, seeing that those which do happen to 

 meet with their complements in the opposite sex help to form a new 

 living individual, and so on through successive generations a(? infinitum. 

 Therefore the reproductive elements of the metazoa and metaphyta are 

 in this respect precisely analagous to the protozoa : potentially^, or in 

 their own nature, they are immortal ; and, like the protozoa, if they die, 

 their death is an accident due to unfavorable circumstances. But the 

 case is quite different with all tbe other parts of a multicellular organ- 

 ism. Here, no matter how favorable the circumstances may be, every 

 cell contains within itself, or in its very nature, the eventual doom of 

 death. Thus, of the metazoa and mtaphyta it is the specialized germ- 

 plasms alone that retain their ijrimitive endowment of everlasting life, 

 passed on continuously through generation after generation of succes- 

 sively perishing organisms. 



So far, it is contended, we are dealing with matters of fact. It must 

 be taken as true that the protoplasm of the uni-cellular organisms and 

 the germ-plasm of the multicellular organisms have been continuous 

 through the time since life first appeared upon this earth ; and although 

 large quantities of each are perpetually dying through being exposed ta 

 conditions unfavorable to life, this, as Weismann presents the matter, 

 is quite a different case from that of all the other constituent parts of 

 multi- eel hilar organisms, which contain within themselves the doom of 

 death. Furthermore, it appears extremely probable that this doom of 

 death has been brought about by natural selection for the reasons 

 assigned by Weismann, namely, because it is for the benefit of all 

 species which perpetuate themselves by sexual methods that their con- 

 stituent individuals should not live longer than is necessary for the 

 sake of originating the next generation and fairly starting it in its own 

 struggle for existence. For Weismann has shown, by a somewhat 

 laborious though still largely imperfect research, that there is through- 

 out all the metazoa a general correlation between the natural life-time 

 of individuals composing any given species and the age at which they 



