336 S. O. MAST 



rapidly than they do later in the cycle; we should find that after a 

 certain time they become sexually mature and are able to conjugate 

 and so to perpetuate the race; and we would find that, ultimately, 

 evidences of weakened vitality and degeneration appear in the aggregate 

 of cells, and that they finally die of old age. 



Calkins consequently holds that conjugation causes an in- 

 crease in the rate of fission and he supports this view by experi- 

 mental evidence w^hich will be presented later. In this conten- 

 tion, however, Calkins stands practically alone. Neither Maupas 

 nor Hertwig, both of whom carried on very extensive experi- 

 ments on various protozoa, were able to find any evidence indi- 

 cating an increase in the rate of fission after conjugation. In 

 fact, Hertwdg actually found a decrease. But both found in their 

 pedigree cultures that the race died out after a certain number of 

 vegetative divisions and they maintain that this must have been 

 due to the absence of conjugation. Consequently they con- 

 cluded that conjugation, although it does not cause an increase 

 in fission-rate, is a rejuvenating process which prevents the race 

 from dying out. 



This view received very serious opposition in the results ob- 

 tained by Woodruff in an extensive series of experiments on 

 Paramecium ('11, '12, '13, '14). These results prove conclusively 

 that certain races of Paramecium can continue to reproduce 

 indefinitely without conjugation. 



Jennings has recently ('13) with characteristic thoroughness, 

 investigated the problem of the significance of conjugation in 

 Paramecium. He maintains (p. 293) that "Conjugation de- 

 creases the rate of fission, causes a great increase in variation 

 in the fission rate, brings about many abnormalities, and greatly 

 increases the death rate." And he concludes (p. 378) 



Conjugation does not produce rejuvenescence, for after conjugation 

 most of the animals are less vigorous than before. What conjugation 

 does is to bring about new combinations of germ plasm, just as is done 

 in the sexual reproduction of higher animals. One result of this is to 

 produce biparental inheritance; another is to give origin to many varia- 

 tions; in the sense of inherited differentiations between different strains. 

 Some of the new combinations are better adapted to the existing conditions 

 than others: these survive while the others die out. 



