100 GENETIC VARIATIONS 



microns (De Garis' race 75). From four pairs of this race 

 there were produced four stocks showing respectively the 

 following mean sizes: 165 microns, 133 microns, 184 microns, 

 136 microns. From his race 3, having a mean length of 198 

 microns, there were two pairs, giving rise to two stocks of 

 having respectively the mean lengths of 70 microns and 203 

 microns. From two ex-conjugant pairs of race 17, having a 

 mean length of 153 microns, were produced two stocks of the 

 following mean sizes: 145 microns, 170 microns. Again, when 

 two races of diverse mean size are repeatedly crossed, stocks of 

 diverse sizes are produced. De Garis made several crosses of 

 his race 39, of 203 microns mean length, with members of his 

 race 11, of 81 microns mean length. Four pairs gave stocks of 

 the following mean lengths, respectively : 144 microns, 191 mi- 

 crons, 207 microns, 91 microns. Crosses between the race 56 

 (mean length 191 microns) and race 83 (mean length 75 

 microns) gave from three pairs stocks of the following mean 

 lengths, respectively: 65 microns, 140 microns, iii microns. 



The different stocks produced by conjugation within a 

 single uniform stock differ greatly in vigor, in rate of multi- 

 plication, and in resistance to unfavorable condition. This 

 has been conspicuous in the investigations of myself and 

 associates on Paramecium aurelia and Paramecium caudatum. 



This production of inherited diversities at conjugation is 

 presumably due to the fact that conjugation, like biparental 

 reproduction in higher forms, is a method of forming many 

 diverse combinations of genes or genie substances. The indi- 

 viduals having the different combinations show very different 

 characteristics, as to size, fission rate, vigor and resistance. 



Furthermore, it can be shown that even during reproduc- 

 tion by fission there may occur at times irregularities of 

 division that result finally in the production and isolation of 

 diverse stocks, differing in hereditary characteristics. For by 



