38 THE DURATION OF LIFE. [I. 



animals and plants, which arose from unicellular forms of life, 

 came to lose this power of living for ever. 



The answer to this question is closely bound up with the 

 principle of division of labour which appeared among multi- 

 cellular organisms at a very early stage, and which has gradually 

 led to the production of greater and greater complexity in their 

 structure. 



The first multicellular organism was probably a cluster of 

 similar cells, but these units soon lost their original homo- 

 geneity. As the result of mere relative position, some of the 

 cells were especially fitted to provide for the nutrition of the 

 colony, while others undertook the work of reproduction. 

 Hence the single group would come to be divided into two 

 groups of cells, which may be called somatic and reproductive 

 — the cells of the body as opposed to those which are concerned 

 with reproduction. This differentiation was not at first abso- 

 lute, and indeed it is not always so to-day. Among the lower 

 Metazoa, such as the polypes, the capacity for reproduction 

 still exists to such a degree in the sc'matic cells, that a small 

 number of them are able to give rise to a new organism, — in 

 fact new individuals are normally produced by means of so- 

 called buds. Furthermore, it is well known that many of the 

 higher animals have retained considerable powers of regenera- 

 tion ; the salamander can replace its lost tail or foot, and the 

 snail can reproduce its horns, eyes, &c. 



As the complexity of the Metazoan body increased, the two 

 groups of cells became more sharply separated from each 

 other. Very soon the somatic cells surpassed the reproductive 

 in number, and during this increase they became more and 

 more broken up by the principle of the division of labour into 

 sharply separated systems of tissues. As these changes took 

 place, the power of reproducing large parts of the organism 

 was lost, while the power of reproducing the whole individual 

 became concentrated in the reproductive cells alone. 



But it does not therefore follow that the somatic cells were 

 compelled to lose the power of unlimited cell-production, 

 although in accordance with the law of heredity, they could 

 only give rise to cells which resembled themselves, and be- 

 longed to the same diflerentiated histological system. But as 

 the fact of normal death seems to teach us that they have lost 



