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



containing two diverse races easily distinguishable b}^ their sizes. 

 Such mixtures were made of L2 + k; L2 + i; D -\- c; D + i;k + ^, 

 and C2 + i" The mixed cultures were kept for months. The two 

 component races remained in every case quite distinct in spite 

 of the common environment. They could easily be distinguished 

 when examined living with low powers of the microscope, and 

 careful measurements and di'awings of preserved material demon- 

 strated the presence of the two races clearly. In the following are 

 given some of the facts and results of specific experiments of 

 this sort. 



On October 8, 1908, specimens of L2 and k were placed together 

 in the same culture. Previous measurements had shown that when 

 these two races are bred separately under as nearly as possible 

 the same conditions, the range of variation for A; is from about 96 

 to 160 microns, with maximum at 128, while for L2 the range is 

 from 176 to 264 microns, with maximum at 208 microns (measure- 

 ments of February 26, 1908). The polygons of variation for these 

 two are shown in fig. 14, at k and L. 



On November 6, after the two had been living mixed together 

 for about a month, a sample of 154 specimens showed a range of 

 variation from 104 to 264 microns, with two maxima at respective- 

 tively 140 and 224 microns, showing that the two races were still 

 present. On March 7, 1909, five months after the two races were 

 mixed, another random sample of 77 specimens was measured. 

 The range of variation was now from 112 to 236 microns, with two 

 maxima at respectively 132 and 220 microns. Fig. 14 shows the 

 polygons of variation for this mixture, in comparison with those 

 for the two races taken separately. 



An examination with the microscope showed clearly the individ- 

 uals of the two races, difi'ering widely in size. Fig. 15 shows a 

 collection of individuals of the two races, which had been living 

 together in the same culture for five months. These illustrate 

 a number of points to which we shall later refer in giving an ac- 

 count of the structural features of the two races. 



In cases where the two races mixed belonged to the same group 

 (caudatum or aurelia) the results were the same. The two races 

 remained quite distinct. A number of such mixtures were made of 



