i9o8] JENNINGS— HEREDITY IN PROTOZOA. 499 



sion, gave, under various different conditions, the following mean 

 lengths in microns (each mean is based on measurements of lOO 

 individuals) : 148.197, 151.920, 158.760, 153.320, 160.852, 156.482. 



It is evident that these means fall in the gap separating the 

 " caudatum" group from the " aurelia" group. I therefore decided 

 to cultivate these under identical conditions with a typical repre- 

 sentative of each of the two main groups. For this purpose I chose 

 D and c, the two lines longest cultivated, which I had used for the 

 study of growth, environmental action, etc. (Tables X., XVIIl., 

 etc.). Twenty-five specimens, each of the three lines, D, c and Nf2, 

 were brought on May i,with the precautions described on page 489, 

 into the same quantity of the same hay infusion and allowed to mul- 

 tiply till May 5. On that date a random sample of each was killed. 

 Though the samples were large, extrinsic conditions prevented my 

 measuring more than the numbers mentioned below ; larger numbers 

 would not have altered the results by more than one or two microns 

 in any case. The mean dimensions of these three lines, cultivated 

 under identical conditions, were 



D (31 specimens), 202.710X51-871 microns. 



Nf2 (33 specimens), 168.970 X 48.970 microns. 



c (43 specimens), 126.605 X 44-930 microns. 



Thus, the dimensions of Nf2 lie almost precisely half way between 

 those of D and c (the dimensions exactly half way between would 

 be 164.658X48.401). We have, therefore, in A^f2 an eighth pure 

 line, intermediate between the '' caudatum " and '' aurelia " groups 

 formed by the other seven. These two groups are then not sepa- 

 rated by an unbridged gap. 



The other character which had been held to se^2ir2ii& Paramecium 

 caudatum from Paramecium aurelia was the presence of but a single 

 micronucleus in the former, while the latter had two. Calkins 

 (1906) showed that in the same pure line we sometimes have two 

 micronuclei, sometimes but one, so that this is not sufficient ground 

 for distinguishing two species. Though the present study has shown 

 that differences in size among different lines are more permanent 

 than the data available to Calkins had seemed to indicate, this does 

 not give any better basis for distinguishing two species, since we 



