158 LIFE AND DEATH. 



the causes which first introduced this period among 1 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 reproduction, 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 sig- 

 nificance of old age. It is obviously of use to man, for it enables 

 the old to care for their children, and is also advantageous in enabling 

 the older individuals to participate in human affairs and to exer- 

 cise an influence upon the advancement of intellectual 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 following 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 encyst- 

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



II. Natural death first appears among the lowest Heteroplastid 

 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 diffe- 

 rentiation took place by the operation of natural selection. 



IV. The fundamental biogenetic law applies only to multi- 

 cellular beings ; it does not apply to unicellular forms of life. 

 This depends on the one hand upon the mode of reproduction by 

 fission which obtains among the Monoplastides (unicellular or- 



