III. 



LIFE AND DEATH. 



In the previous essay, entitled ' The Duration of Life,' I have 

 endeavoured to show that the hmitation of Hfe in single indivi- 

 duals by death is not, as has been hitherto assumed, an inevit- 

 able phenomenon, essential to the very nature of life itself; but 

 that it is an adaptation v^hich first appeared when, in conse- 

 quence of a certain complexity of structure, an unending life 

 became disadvantageous to the species. I pointed out that we 

 could not speak of natural death among unicellular animals, for 

 their growth has no termination which is comparable with 

 death. The origin of new individuals is not connected with 

 the death of the old ; but increase by division takes place in 

 such a way that the two parts into which an organism separates 

 are exactly equivalent one to another, and neither of them is 

 older or younger than the other. In this way countless num- 

 bers of individuals arise, each of which is as old as the species 

 itself, while each possesses the capability of living on inde- 

 finitely, by means of division. 



I suggested that the Metazoa have lost this power of un- 

 ending life by being constructed of numerous cells, and by the 

 consequent division of labour which became established be- 

 tween the various cells of the body. Here also reproduction 

 takes place by means of cell-division, but every cell does not 

 possess the power of reproducing the. whole organism. The 

 cells of the organism are differentiated into two essentially dif- 

 ferent groups, the reproductive cells— ova or spermatozoa, and 

 the somatic cells, or cells of the body, in the narrower sense. 

 The immortality of the unicellular organism has only passed 

 over to the former ; the others must die, and since the body of 

 the individual is chiefly composed of them, it must die also. 



I have endeavoured to explain this fact as an adaptation to 

 the general conditions of hfe. In my opinion life became limited 



