122 GENETIC VARIATIONS 



After conjugation, the individual A contains a nucleus that 

 was half derived from its mate B) and this half nucleus from 

 B changes the characteristics of the descendants of A. But to 

 make this change may require a very considerable time, dur- 

 ing vv^hich the individual A and its descendants divide many 

 times, giving rise to many successive generations of offspring. 

 All this has been demonstrated for the infusorian Paramecium 

 by the researches of Sonneborn and Lynch,^ and those of 

 De Garis." To follow through a typical case of size inheritance 

 will give the best conception of what happens. We will take 

 a case described by De Garis. He crossed an individual A of 

 a very large race of Paramecium caudatum, having an aver- 

 age length of 198 microns, with an individual 5 of a very 

 small race, having an average length of but 73 microns (see 

 Figure 19). The two exchanged half nuclei and separated, 

 each retaining its original length. Each began to divide, about 

 once a day. As the generations passed, the descendants of A 

 began to grow smaller, the descendants of B began to grow 

 larger. Our figure shows the successive changes in size for 

 two-day periods, to the twenty-second day after conjugation. 

 By this time the two had reached practically the same size. A 

 similar course was followed in other cases of crosses between 

 individuals of different size, save that in some cases an even 

 longer time was required to reach the final size, equal in the 

 descendants of the two mates. In the most extreme cases, 

 thirty-six days were required to reach the ultimate size. In the 

 inheritance of fission rates, the new nuclear combination pro- 

 duces its final effect much more quickly. In some cases the 

 fission rate was changed at once after conjugation; within the 

 first five-day period the slow race had begun to divide more 

 rapidly, the fast race more slowly, so that the two were equal. 

 In other cases there is apparently a lag like that which occurs 

 in the inheritance of size, but not lasting so long. 



