XII.] CONJUGATION AND SEXUAL REPRODUCTION. 207 



changes which lead to conjugation, which could perhaps only 

 have been prevented by special means '. 



Therefore we cannot speak of natural death as an adaptation 

 to prevent unconjugated individuals from gaining the upper 

 hand ; and in any case natural death cannot be admitted to 

 obtain among Infusoria in general,, inasmuch as it only occurs in 

 those animals which are abnormal in not attaining to conjugation. 



We need not discuss whether the dying out of the uncon- 

 jugated animals in an Infusorian colony, is an adaptation, 

 specially intended for the removal of these harmful individuals, 

 or whether, as I prefer to assume, it follows as a consequence 

 of those changes which are preparatory to pairing. But even 

 the former assumption affords no support to Maupas ; because 

 the natural death presupposed by him is the ver}^ reverse 

 of an adaptation, being a fundamental attribute of life itself, — 

 the inherent tendency to wear itself out. According to this 

 view, Infusoria are predestined to death ; they can however be 

 rescued by the magic of conjugation, and thus acquire a new 

 span of life. 



Such a view does not admit of direct refutation ; we can 

 only show that it has its origin in the old mystic conception of life, 

 and that it is superfluous. 



Conjugation was long spoken of as the ' sexual reproduction ' 

 of Infusoria before we had a more intimate knowledge of the 

 nature of the process. The ' tertium comparationis ' was that fusion 

 of two cells into one which occurs at any rate in the original 

 form of both fertilization and conjugation. I have been accus- 

 tomed for many years to urge, in my lectures, that conjugation 

 is not reproduction, but rather its opposite ; for reproduction 

 implies an increase of at least one in the number of individuals, 

 while conjugation leads to a decrease, two individuals fusing 

 into one. It has long been recognized that the processes which 

 take place in conjugation and fertilization have in themselves 

 nothing to do with reproduction. Maupas admits this and ex- 

 presses it quite clearly and correctly when he states that 



^ I am here referring to the interesting facts discovered by R. Hertwig, 

 which he explained as an Infusorian parthenogenesis. The subject is 

 not, however, sufficiently mature for further consideration in this place. 

 See R. Hertwig, ' Ueber die Conjugation der Infusorien.' Munich, 

 1889. 



