III. 



LIFE AND DEATH. 



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

 endeavoured to show that the limitation of life in single individuals 

 by death is not, as has been hitherto assumed, an inevitable phe- 

 nomenon, essential to the very nature of life itself; but that it is 

 an adaptation which first appeared when, in consequence 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 termina- 

 tion which is comparable with death. The origin of new indivi- 

 duals 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 numbers of individuals arise, each of which is as old as 

 the species itself, while each possesses the capability of living on 

 indefinitely, by means of division. 



I suggested that the Metazoa have lost this power of unending 

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

 division of labour which became established between 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 different 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 life. In my opinion life became limited in 



