weismann's theory of heredity. 445 



multicellular organisms. !N"owas before stated, dift'ereut forms of uni- 

 cellular organisms are supposed to have originated as so many results 

 of differences in the direct action of the environment. Consequently, 

 according to the theory, all congenital variations which now occur in 

 multi cellular organisms are really the distant results of variations that 

 were aboriginally induced in their uni-cellular ancestors by the direct 

 action of surrounding conditions of life. 



I think it will be well to conclude by briefly summarizing the main 

 features of this elaborate theory. 



Living material is essentially, or of its own nature, imperishable, 

 and it still continues to be so in the case of unicellular organisms which 

 l)ropagate by fission or gemmation. But as soon as these primitive 

 methods of propagation became, from whatever cause, superseded by 

 sexual, it ceased to be for the benefit of species that their constituent 

 individuals should be immortal, seeing that, if they continued to be 

 so, all species of sexually-reproducing organisms would sooner or later 

 come to be composed of broken down and decrepit individuals. Con- 

 sequently, in all sexually-reproducing or multi-cellular organisms, nat- 

 ural selection set to work to reduce the term of individual life-times 

 within the narrowest limits that in the case of each species are com- 

 patible with the procreation and the rearing of progeny. Nevertheless, 

 in all these sexually-reproducing organisms the primitive endowment 

 of immortality has been retained with respect to their germ-plasm, 

 which has thus been continuous, through numberless generations of 

 perishing organisms, from the first origin of sexual reproduction till 

 the present time, ^ow it is the union of germ-plasms which is required 

 to reproduce new individuals of multi-cellular organisms that determines 

 congenital variations on the part of such organisms, and thus furnishes 

 natural selection with the material for its work in the way of organic 

 evolution, — work therefore which is impossible in the case of uni-cel- 

 lular organisms, where variation can never be congenital, but always 

 determined by the direct action of surrounding conditions of life. 

 Again, as the germ-plasm of multi-cellular organisms m continuous from 

 generation to generation, and at each impregnation gives rise to a more 

 or less novel set of congenital characters which are of most service to 

 the organisms presenting them, is really or fundamentally at work 

 upon those variations of the germ-plasm which in turn give origin to 

 those variations of organisms that we recognize as congenital, there- 

 fore, natural selection has always to wait and to watch for such varia- 

 tions of germ-plasm as will eventually prove beneficial to the individuals 

 developed therefrom, who will then transmit this peculiar quality of 

 germ-plasm to their progeny, and so on. Therefore also — and this is 

 most important to remember — natural selection as thus working be- 

 comes the one and only cause of evolution and the origin of species in 

 all the multicellular organisms, just as the direct action of the environ- 

 ment is the one and only cause of evolution and the origin of species 



