24> Life and Death, Heredity and Evolution 



sands of specimens have been produced from a single one. 

 But as this goes on, they gradually lose their vitality, get 

 old and decrepit, and must all in the course of time die, 

 unless mating intervenes to save them. On the question of 

 just what form this aging and decrepitude takes and just 

 what are its symptoms, there was much difference of opinion. 

 Biitschli and Balbiani believed that the power of reproduc- 

 tion gradually became less and less, so that division into 

 two became less frequent; this idea was accepted by many, 

 and has been maintained in recent years by Calkins; it is 

 perhaps the prevalent form of the theory. On this view the 

 occurrence of conjugation restored the reproductive power, 

 so that fission now continued as rapidly as it did before the 

 animals grew old. On the other hand, though the fact is 

 not generally realized, some of the chief upholders of the 

 theory of rejuvenescence emphatically rejected this idea 

 that aging showed itself in a decline of the rate of repro- 

 duction ; this is notably the case with Maupas, who asserted 

 positively that no such decline occurred before conjugation, 

 and that after conjugation reproduction was no more ener- 

 getic than before; and Richard Hertwig discovered experi- 

 mentally that Maupas' statement is correct. They believed 

 however that in other ways the animals become decrepit and 

 that they must die if conjugation did not occur. 



Now, just what is it that happens in mating which might 

 be conceived to bring about rejuvenescence? There are two 

 main things that occur in conjugation; to get these clearly 

 in mind we must look for a moment at the structure of an 

 infusorian, and at the process of conjugation. 



An infusorian resembles in the plan of its structure a 

 single cell of a higher organism, but has its nucleus in two 

 parts (Figure 7, a). One of these parts is large and seems 

 to be the active portion ; the other is very minute and appears 



