io PROBLEMS OF EVOLUTION 



viduals thus produced are weak and senile I infer this from 

 what has been proved in the case of other unicellular organisms. 

 Vigorous life and continued fission are no longer possible to the 

 amoeba except on one condition : it must fuse or conjugate with 

 another amoeba, and then rejuvenescence will follow. 1 The 

 interchange of matter between the two nuclei is a lengthy pro- 

 cess, during which many generations might have been born by 

 fission. It seems, therefore, a priori probable that conjugation 

 intervenes, because without it further multiplication is impos- 

 sible. But there is direct evidence. M. Maupas succeeded in 

 isolating individual infusorians (one-celled animals moving by 

 means of small hair-like structures called cilia) : a single re- 

 presentative of a species was set apart, and the number of 

 fissions which took place were counted. In some cases they 

 amounted to as many as 600. But in every instance there 

 came at length a time when the organisms were unable to 

 continue reproducing themselves by this means ; they encysted 

 or died of atrophy, being no longer able to nourish them- 

 selves. But if, when the power of fission was coming to an 

 end, one was removed from the rest and put with one of 

 another stock, conjugation took place. The two individuals 

 united, and there was an interchange of nuclear matter, which 

 is the essential part of the process. This is effected by that 

 part of the nucleus which is called the micro-nucleus and which 

 couples with the corresponding part in the other individual. 

 After the interchange the two infusorians separate, and after 

 some interval fission begins again. That conjugation brings 

 about rejuvenescence, there is every reason to believe. The 

 reduction in the size of the micro-nucleus when the power of 

 fission comes to an end seems to be a sign of senility. In some 

 cases even external characters (among them the mouth, the most 

 important of them all) was found to be very imperfectly developed. 

 It may possibly be objected that the conditions were unnatural, 

 the specimens being kept in a small amount of water under glass. 

 But they were carefully supplied with their proper food, animal 

 in some cases, vegetable in others, and seem to have enjoyed 



1 Some authorities deny that conjugation takes place among amoeba:. See p. 34. 



