l6o LIFE AND DEATH. [III. 



pointed out that ' death is not always preceded by senihty or 

 a period of old age \' 



The materials are wanting for a comprehensive investigation 

 of the causes which first introduced this period among the 

 higher Metazoa ; in fact the most fundamental data are absent, 

 for we do not even know the part of the animal kingdom in 

 which it first appeared : we cannot even state the amount by 

 which the duration of life exceeds that of the period of repro- 

 duction, or what is the value to the species of this last stage in 

 the life of the individual. 



It is in these general directions that we must seek for the 

 significance of old age. It is obviously of use to man, for it en- 

 ables the old to care for their children, ;;and is also advantageous 

 in enabling the older individuals to participate in human affairs 

 and to exercise an influence upon the advancement of intel- 

 lectual powers, and thus to influence indirectly the maintenance 

 of the race. But as soon as we descend a step lower, if only as 

 far as the apes, accurate facts are wanting, for we are, and shall 

 probably long be, ignorant of the total duration of their life, and 

 the point at which the period of reproduction ceases. 



I must here break off in the midst of these considerations, 

 rather than conclude them, for much still remains to be said. 

 I hope, nevertheless, that I have thrown new light upon some 

 important points, and I now propose to conclude with the fol- 

 lowing short abstract of the results of my enquiry. 



I. Natural death occurs only among multicellular beings ; it 

 is not found among unicellular organisms. The process of en- 

 cystment in the latter is in no way comparable with death. 



II. Natural death first appears among the lowest Hetero- 

 plastid Metazoa, in the limitation of all the cells collectively to 

 one generation, and of the somatic or body-cells proper to a 

 restricted period : the somatic cells afterwards in the higher 

 Metazoa came to last several and even many generations, and 

 life was lengthened to a corresponding degree. 



III. This limitation went hand in hand with a differentiation 

 of the cells of the organism into reproductive and somatic cells, 

 in accordance with the principle of division of labour. This 

 differentiation took place by the operation of natural selection. 



^ See the first essay on ' The Duration of Life,' p. 22. 



i 



