FERTILIZATION IN THE LIFE-HISTORY 41 



ished vigor, and he points out that we cannot therefore 

 assume, ''as has been done by some authors, that if the 

 animals continue to multiply after conjugation, this 

 shows that conjugation has had a rejuvenating effect, 

 for the same specimens continue equally without con- 

 jugation." The phenomenon of endomixis was not 

 known at the time that this was written, and Jennings 

 therefore did not determine its occurrence in the sepa- 

 rated conjugants; but it is reasonable to assume that it 

 occurred soon, if not immediately, because it is apparently 

 a normal process in all races of Paramecium. He con- 

 cludes from his very extensive, well-controlled series 

 of experiments that "there is no evidence that conjuga- 

 tion in the infusoria increases the reproductive power, 

 or rejuvenates the organism physiologically in any way" 

 (Jennings, 1913a). 



In these experiments Jennings demonstrated that 

 conjugation, on the average, decreases the rate of fission 

 very greatly instead of increasing it; it produces a 

 great amount of variation in this respect, ranging from 

 a total loss of capacity for fission to a rate nearly equal 

 to the original, and also variations in other character- 

 istics. "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 give origin to many variations, in the sense of inher- 

 ited 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" (Jennings, 1913^). 



In respect to conjugation, Paramecium and the cili- 

 ates generally are quite aberrant in comparison with other 



