180 On the Genetics of the CUktte Protozoa 



— and even necessaiy — to look for periods of adolescence, maturity, 

 senescence, and death, in the successive generations of individiuils 

 derived from an exconjugant. But as soon as it is realized that a 

 ciliate is a non-cellular but complete organism, homologous with 

 a whole metazoan ; as soon as it is realized that the only resemblance 

 between a ciliate and a metazoan cell was created by biologists when 

 they gave the name " cell " to both : then it will also be realized that 

 there is no reason to expect that successive generations of ciliate 

 individuals will manifest the same series of phenomena as is manifested 

 by a single individual metazoon during its life-time. One of the chief 

 results of the researches recorded in previous chapters is the demonstra- 

 tion that events, whose occurrence we have absolutely no reason to 

 expect, do not, in fact, occur. 



126. It has been shown (§ 48) beyond all reasonable doubt that 

 under suitable conditions ciliates are able to live and multiply, in their 

 own fashion, for an unlimited time' — like all other organisms that are 

 well adapted to their environment. To ask whether they become 

 "senile" m the course of successive generations, whether "protoplasmic 

 old age " sets in, whether " rejuvenation by fertilization " is a periodic 

 necessity, and so on — to ask such (juestions is to propound problems 

 which are either unanswerable or meaningless. Among ciliates, off- 

 spring are formed by the division, growth and differentiation of the 

 protoplasm of their parents — as in all other organisms. If "immortality " 

 can be predicated of the ciliates, it can also be predicated of all other 

 organisms in the same sense. "Are the descendants of a ciliate older 

 than their progenitor ? " is the same question as " Is the child older 

 than its father?" There is no jjroblem here at all, for the answer is 

 self-evident as soon as the (juestioner intimates what he means by "old." 



127. That conjugation is able to "rejuvenate" is a belief whose 

 origin is revealed in the word " fertilization." Conjugation is accom- 

 panied by " fertilization," and has consequently become confounded with 

 the original connotation of that term". Probably no living biologist 

 would care to advocate the view that the cytological phenomenon now 

 called " fertilization " is a process of " revitalizing the germ " — though 

 more than one can be found to discuss whether "conjugation results in 

 rejuvenescence." But quite apart from any mental or verbal tangles 



1 I believe there are no a priuri reasons, or arguments from analogy with other non- 

 cellular organisms, which indicate that the ciliates arc incapable of continuing to multiply 

 asexually— in favourable circumstances— for an unlimited number of generations. 



^ This was, of course, pointed out long ago by Weismann. 



