THE DURATION OF LIFE. 27 



nothing comparable to that deterioration of the body which takes 

 place in the higher organisms. Unicellular animals are too simply 

 constructed for this to be possible. If an infusorian is injured by 

 the loss of some part of its body, it may often recover its former 

 integrity, but if the injury is too great it dies. The alternative is 

 always perfect integrity or complete destruction. 



We may now leave this part of the subject, for it is obvious that 

 normal death, that is to say, death which arises from internal 

 causes, is an impossibility among these lower organisms. In those 

 species at any rate in which fission is accompanied by a circulation 

 of the protoplasm of the parent, the two halves must possess the 

 same qualities. Since one of them is endowed with a potentiality 

 for unending life, and must be so endowed if the species is to persist, 

 it is clear that the other exactly similar half must be endowed 

 with equal potentiality. 



Let us now consider how it happened that the multicellular 

 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 multicellular 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 homogeneity. 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 op- 

 posed to those which are concerned with reproduction. This 

 differentiation was not at first absolute, 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 somatic 

 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 regeneration ; 

 the salamander can replace its lost tail or foot, and the snail can 

 reproduce its horns, eyes, etc. 



