498 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



as 116.856, and these were correlated with corresponding measure- 

 ments throughout the series. 



These facts, of course, do not show that the size may not change 

 at the time of conjugation or before or after. What they do show 

 is that any differences thus produced do not account for the perma- 

 nent differentiations we have found among different lines. We may 

 distinguish (i) differences in size due to growth; (2) those due to 

 nutrition and other environmental conditions; (3) those due to 

 different stages in the life cycle (as a rule not marked in comparison 

 with the others) ; (4) inherent, hereditary differences in size, per- 

 sisting when all other conditions are made the same. 



(d) Lines Intermediate Betzveen the Tzvo Main Groups. The 

 Question of Species in Paramecium. 



As we have already noted, the seven differentiated lines which 

 we have thus far distinguished fall into two main groups, separated 

 by a wide interval. In Table XXV. we find one group with mean 

 lengths varying from 119.200 to 140.800 microns, while in the other 

 group the mean lengths vary from 176.901 to 220.560 microns. 

 Between the two there is thus a gap of 36.101 microns in which none 

 are found. Is this gap constant and characteristic, so that our two 

 large groups are permanently differentiated? If so, we should have 

 some real basis for the common distinction into two species, Para- 

 mecium caudatuni (larger) and Parameciiini aurelia (smaller). The 

 fact that we find in nature such cultures as that shown in Table 1. 

 (page 398), in which the individuals are distinctly separated into the 

 two groups, seems to raise a presumption that the groups are natural 

 ones, not due to accidents of selection. 



For a long time I found no pure lines that were intermediate 

 between these groups. It is possible that this was partly due to a 

 tendency to choose for breeding the largest and smallest specimens, 

 rather than intermediate ones, since my purpose at first was to deter- 

 mine whether there were any permanent differentiations at all; for 

 this, marked differences were desirable. 



In the course of work on certain problems connected with con- 

 jugation, I came in possession of a pure line, Nf2, descended from a 

 single ex-conjugant. This, when cultivated in the usual hay infu- 



