IMMORTALITY OF THE PROTOZOA 27 



quantitatively much inferior to the latter. Nageli's conception 

 of the nature of the idioplasm difiers somewhat from Weismann's 

 conception of the nature of the germ-plasm, but the idea of 

 two distinct kinds of living substance is common to both. 

 Weismann distinguishes the two as germ-plasm and somato- 

 plasm. 



The conception at the basis of Weismann's theory is that ot 

 the continuity of the germ-plasm. The germ-plasm, like the 

 Protozoa, is immortal. It does not die with the life of the indi- 

 vidual, but is continued in an unending chain from one genera- 

 tion to another. When we speak of immortality in connection 

 with the germ-plasm, or in connection with the Protozoa, the 

 word must not be taken in any metaphorical sense. The uni- 

 cellular organisms do not succumb to the " wear and tear " of 

 life, to natural death, as the higher organisms do. Having 

 reached a certain limit of growth, beyond which they cannot go 

 without dividing, they divide. The two halves separate as 

 independent units, and go through a similar process of growth 

 and division, and so on for an unlimited time. Unicellular 

 organisms are, therefore, potentially immortal — that is to say, 

 they do not cease to exist as a result of natural death. There is 

 no such thing as natural death among the Protozoa. 



It is essentially the same with the germ-plasm. The simple 

 fact that the excision of the genital organs prevents the indi- 

 vidual on which it is practised from reproducing itself, and 

 reduces it to sterility, shows that the germ-plasm cannot be 

 made afresh from somato-plasm ; and it shows also that certain 

 cells only are capable of reproduction, are capable, consequently, 

 of containing germ-plasm in an inactive condition. 



For the germ-plasm is present in every organism in two forms 

 — active and inactive. Active germ-plasm is that part of the 

 germ-plasm which goes to build up the bodily frame or soma of 

 the individual. For this reason it is called " somatic idioplasm." 

 It is that portion of the germ-plasm set apart for individual 



