30 EVOLUTION OF LIVING ORGANISMS. 



from such undifferentiated cells set apart for the purpose 

 of reproduction, sometimes from the very earliest stages 

 of embryonic development. They may be traced back, 

 and occasionally even be distinguished under the micro- 

 scope through an unbroken lineage of embryonic cells to 

 the fertilised ovum. Weismann's famous theory of the 

 continuity of the germ-plasm is founded on these facts. 

 According to it, the germ-plasm, that special protoplasm 

 of the gametes which is handed on by them from genera- 

 tion to generation, and gives rise to new individuals, is in 

 a sense independent of the body or soma in which it 

 develops. Whereas the rest of the multicellular organism, 

 the soma, undergoes differentiation and dies, the germ- 

 cells continue for ever giving rise to new generations, the 

 germ-plasm passing from parent to offspring. This brings 

 us to the consideration of the origin and biological sig- 

 nificance of death. 



Is death an essential inevitable attribute of life ? If 

 by death we mean mere decay, mere katabolic changes, 

 the answer is yes. In this sense we begin to die as soon 

 as we are born. But this is not what is meant by death 

 in ordinary language. We do not speak of death unless 

 there is a corpse. And the question is, putting aside ac- 

 cidental death through violence or disease : does natural 

 death occur always, or does it even occur at all ? Would 

 an organism shielded from all unfavourable conditions 

 continue to live for ever, or is it wound up, so to speak, to 

 live for only a limited time ? It has already been men- 

 tioned that among the bacteria and unicellular organisms 

 propagation by a process of fission and spore formation 

 may go on indefinitely if the conditions are favourable. 

 True it is that the individual may be said to disappear in 

 the splitting ; but death can have no sting for an organ- 

 ism which divides into two living halves. Even when 

 sexual union is necessary no corpse need be produced. 



Death, then, appears among living organisms when a 

 soma or body becomes differentiated from the germ-cells. 



