Races of Paramecium. 509 



caudatum/' and if this circumlocution implies that one has been 

 shown to change into the other, it is actually misleading. 



Part 2. Other Characteristics of the Diverse Races 



H. S. JENNINGS 

 1. Differences in size and their permanence 



The two pure lines or races that have been longest in the labora- 

 tory are those designated by D and c in the earlier paper (Jennings, 

 '08). Each was derived from a single individual; D from a large 

 specimen isolated April 12, 1907, c from a small specimen isolated 

 April 9, ] 907. These have thus been in the laboratory at the pres- 

 ent time (June, 1910) more than three years. At the first n;ieasure- 

 ment the progeny of D had a mean length of 182.76 microns; 

 those of c a mean length of 130.12 microns (I.e., pp. 404, 405). 

 Throughout the paper of 1908 many measurements of D and c 

 are given, taken at intervals from June 11, 1907, to March 19, 

 1908, The length for D was found to vary under different envi- 

 ronments, from 146.11 to 202.28 microns; that for c from 99.67 

 to 158.8 microns. Under the same environment the race c always 

 ranged about 50 microns shorter than the race D. The most re- 

 cent measurement of these two races, taken April 21, 1910, more 

 than three years after they were first isolated and after they had 

 been kept for seven weeks with the greatest care under identical 

 conditions, give D a length of 162.28 microns; c of 109.25 microns. 

 Thus the difference between the two races is quite permanent. 

 D, as we have seen, belongs to the caudatum grouj., having a 

 single micro-nucleus; c to the aurelia group, with two micro-nuclei. 



Measurements of other races at long intervals show similar 

 constancy in the differences in size, even though the animals are 

 kept for long periods with the most rigid care under the same con- 

 ditions, nutritive and otherwise. This is as true of the races 

 distinguishable within each of the two main groups (caudatum or 

 aurelia), respectively, as of those belonging to different groups. 

 This may be shown in brief in the following table, giving the meas- 

 urements at diverse periods of the races most studied. Where the 



