261 



tains a fraction of the parental living substance. As this pro- 

 cess of division has gone on from generation to generation it is 

 clear that the protoplasm of the ancestral and primeval Amoeba 

 has lived on ever since Amoeba-like organisms made their appear- 

 ance. The Protozoa then, as also the Protophyta, are endowed 

 with the potentiality of eternal life. This does not of course 

 imply that they cannot die, or that they do not die, but merely 

 that, if they be shielded from fatal accidents, they do not die a 

 natural death, but live on, increasing in size up to a certain 

 limit, and when that limit is reached dividing into two or more 

 similar organisms. And we may go as far as to say that every 

 Protozoon of the present day is alive with the actual life of its 

 primeval ancestor, the body of which has thus lived on in the 

 substance of its descendants ever since life itself made its ap- 

 pearance on this planet. 



Metazoa and Metaphyta on the other hand do die a natural 

 death. To the hisfher orofanisms with which we are most familiar 

 death is undoubtedly the end of all things. 



These multicellular beings have undoubtedly descended from 

 unicellular organisms which, we have seen, are endowed with the 

 potentiality of everlasting life, and the former must have developed 

 the power of dying when they ceased to consist of simple cells. 

 The question thus arises why should life, imm ^rtal in the case of 

 the unicellular organisms, have ceased to be so in the case of the 

 multicellular ? 



Weismann's answer is based upon the fact that multicellular 

 organisms do not propagate themselves solely by such a-sexual 

 methods as fission and gemmation, but by a sexual jDrocess in 

 which the origin of a new individual depends upon the fusion or 

 €onjugation of certain specialised portions of two parents. 



It is true that the general body-cells of multicellular organ- 

 isms can reproduce themselves to a certain degree, as shown in 

 the growth and repair of tissues and organs, or even in certain 

 cases of whole members, but it is only a certain specialised group 

 of these cells which can do so to that unlimited extent which is 

 characteristic of unicellular organisms. 



It is to this limitation in respect of the po A^er of division of 

 the somatic- or body-cells that the phenomenon of death is owing. 



Thus we return to the cardinal point which is so important. I 

 repeat it at the risk of being tedious, that the cells of the complex 

 Metazoon can be divided into two categories, reproductive cells 

 and body- or somatic-cells, the former inheriting from their ances- 

 tors, the Protozoa, their unlimited power of division, the latter 

 possessing this power only to the extent of the attainment of the 

 limit of size of the individual, and when this limit is attained the 



