38 PROBLEMS OF FERTILIZATION 



senescence ending in death. The ex-conjugants are, 

 however, rejuvenated and begin new life-cycles. 



Maupas thus distinguished periods of youth, sexual 

 maturity, and old age in the ciliate life-cycle; and he 

 believed that conjugation brought about a complete 

 renewal of youth. He believed, moreover, that diverse 

 ancestry of the conjugating individuals was important 

 for rejuvenation, and that union of too closely related 

 individuals was likely to be abortive in this respect. 



These views stood in the sharpest contrast to those 

 of Weismann, who held "that the deeper significance 

 of every form of amphimixis whether occurring in 

 conjugation, fertilization, or in any other way consists 

 in the creation of that hereditary individual variability 

 which is requisite for the operation of the process of 

 selection, and which arises from the periodical mingling 

 of the individually different hereditary substances.' 

 Moreover, Weismann was not simply content to empha- 

 size this aspect of conjugation, but he also rejected and 

 ridiculed the rejuvenation conception: "To my mind it 

 is difficult to understand how an almost exhausted vital 

 force could be raised again to its original state of activity 

 as the consequence of a union with another equally 

 exhausted force. ' Of course such a proposition gains 

 its strength from its form rather than from its sub- 

 stance. 



To explain the death of individuals that fail to con- 

 jugate, Weismann supposes that, when the long prepared 

 period of conjugation approaches, the appropriate nuclear 

 maturation processes occur; the result of such matura- 

 tion is to organize the individuals as gametes, which die 

 like any gametes which fail to unite. 



