182 THEORY OF EVOLUTION 



middle, each half replacing its lacking part. 

 Both the small nucleus (micronucleus) and the 

 large nucleus (macronucleus) divide at each 

 division of the body. Jennings found that 

 while individuals descended from a single 

 paramecium vary in size (fig. 89), yet the 

 population from a large individual is the same 

 as the population derived from a small individ- 

 ual. In other words, selection produces no re- 

 sult and the probable explanation is, of course, 

 that the different sizes of individuals are due 

 to the environment, while the constancy of the 

 type is genetic. Jennings found a number of 

 races of paramecium of different sizes living 

 under natural conditions. The largest indi- 

 vidual of a small race might overlap the small- 

 est individual of other larger races (fig. 89) ; 

 nevertheless each kind reproduced its particu- 

 lar race. The results are like those of Johann- 

 sen in a general way, but differ in that repro- 

 duction takes place in paramecium by direct 

 division instead of through self-fertilization as 

 in beans, and also in that the paramecia were 

 probably not homozygous. Since, however, 

 so far as known no "reduction" takes place in 



