VI 



What are the Results of Mating? Rejuvenescence and 

 Mating. Heredity and Variation, and Mating. 



WE have so far tried to judge of the nature and effects 

 of mating chiefly by examining the organisms or 

 parts that unite, and the processes that occur in union. 

 Now we turn to the study by observation and experiment of 

 the effects of mating. Does mating actually cause re- 

 juvenescence, in the sense of an increased vigor and vital- 

 ity? The chief definite criterion for such increased vigor 

 that has been proposed is, that after mating, reproduction 

 should take place more energetically than before. In the 

 Protozoa, for example, old age is held to show itself by a 

 decrease in the rate of multiplication; rejuvenescence by 

 an increase. This is the theory which Calkins in recent 

 times has made peculiarly his own. "As with the metazoon 

 so with the aggregate of Protozoa cells, we note a period 

 of youth characterized by active cell proliferation; this in 

 both groups of organisms is followed by the gradual loss 

 of the division energy accompanied by morphological 

 changes in type of the cells preliminary to conjugation and 

 fertilization and to the renewal of vitality by this means" 

 (Calkins, 1906, p. 232). This appears to be what is com- 

 monly understood by the theory of rejuvenescence. 



How can we find out whether this theory is correct? Evi- 

 dently the direct method is to take a stock of infusoria, such 

 as Paramecium, and allow it to multiply till it gets to the 



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