328 THE EVOLUTION THEORY 



reproihiction,' and try to probe more deeply into the meaning of its 

 establishment : in the meantime I must restrict myself to having 

 shown its significance in the union of tlie hereditary substances of 

 two indi\iiluals, and at the same time to controverting the theory 

 of the ' rejuvenating power ' of amphimixis. I use this expression in 

 its original sense, which indicates that every life is gradually wearing 

 itself away and would become extinct were it not fanned to flame 

 again by amphimixis — by an artifice of Nature, we may say. This 

 conception rests on the fact that the cells of the multicellular body 

 possess for the most part only a limited length of life, for they are 

 used up by the processes of life, and they break up ai(id die, some 

 sooner, some later. As it is observed that all true somatic cells, 

 among higher animals at least, are subject to this law of mortality, 

 but that the germ-cells are not, and that, furthermore, the germ-cells 

 only develop when they are fertilized, the cause' of the potential 

 immortality of the germ-cells is believed to lie in amphimixis, and 

 a ' rejuvenating ' power in fertilization, or, more generally, in amphi- 

 mixis, is inferred. Mystical as this sounds, and little as it agrees 

 with our otherwise mechanical conceptions of the economy of life, 

 it was until very recently a widespread view, although perhaps it is 

 now abandoned by many who formerly held it, and has been im- 

 perceptibly modified into a quite different conception, for which the 

 word 'rejuvenescence' is retained, but with the altered meaning of a 

 mere ' strengthening of the metabolism ' or ' of the constitution.' By 

 many authors, indeed, the two meanings of the word are not clearly 

 kept apart. I shall return later to the modified meaning of the word 

 ' rejuvenescence/ and shall keep in the meantime to the original 

 meaning of the word, which implies a renewal of life which would 

 otherwise die out. 



This meaning seemed to gain a firm hold, wdien, about fifteen years 

 ago, the French investigator Maupas published his remarkable obser- 

 vations on the conjugation of Infusorians. These seemed to show 

 that colonies of Infusorians wdiich w^ere artificially prevented from 

 conjugating gradually died out ; not of course at once, but after 

 many, often several hundred, generations; ultimately a degeneration 

 of all the animals in such colonies set in, and ended only with their 

 utter extinction. Maupas himself interpreted this as a senile de- 

 fieneratlon which took place because conjugation had been prevented, 

 and he therefore regarded conjugation as a ' rajeunissement karyo- 

 ga7)iique,' a rejuvenescence, and therefore a means of preventing the 

 ageing and final dying off of the individuals — of obviating, in short, 

 the natural death to which in his opinion they would otherwise be 



