520 H. S. Jennings and George T. Hargitt. 



side for months. Fig. 17 shows typical specimens of the two races 

 from a culture in which they had been living together for three 

 weeks. 



Mixtures of other races, of Z) + i, of D + c, of d + i, of L2 

 + i, were made and the progress of the cultures observed for 

 weeks or months. In every case the two races remained evidently 

 distinct ; in an examination under the microscope the two diverse 

 sizes were very noticeable. It was not thought necessary to make 

 precise measurements and figures, save for the two typical mix- 

 tures above described. 



A question that perhaps requires brief discussion is that regard- 

 ing the relation of these diverse sizes to conjugation.^ Is it abso- 

 lutely clear that these ''races" of diverse size are not really 

 diverse merely because they represent different periods in the life 

 cycle, from conjugation to conjugation? To this question an 

 affirmative answer can be given; it is absolutely clear that the 

 differences are not due to the period of the cycle, for mamj of 

 the races have gone through several or rnany complete "cycles'' in 

 the laboratory, without changing their relative sizes. The race k 

 has gone through at least twenty such cycles; the races C2 and g 

 through several ; the races c and Lo through two ; the race i through 

 at least one — and at no time has there been any change in the 

 relative size of these races. Fig. 18 shows the relative sizes of 

 certain races at conjugation, when living in hay culture. The 

 pairs of k and i in this figure were living together in the same cul- 

 ture; they therefore show the relative size at conjugation under 

 identical conditions. 



It is clear therefore that the differences in size among the differ- 

 ent races are independent of environmental influence — the differ- 

 ences persisting when the organisms are kept for long periods in 

 the same environment. It is further clear that the differences are 

 permanent, at least for a period exceeding three years, and that 

 they are not dependent upon the relative period in the life cycle 

 of the animals from conjugation to conjugation. 



' This question was taken up in my paper of 1908; further data are here added. 



