PHYSIOLOGICAL VARIATIONS IN PARAMECIUM 513 



be characterized by the particular line that chanced to be in the 

 ascendancy at the tune of killing and measuring. According to 

 our results such an ascendant line might be like our J 20, or 17, 

 or 10, or 8 or 15, each of which would have given decidedly differ- 

 ent results from the others and yet all of them would have been 

 possibilities, had we maintained all the progeny of the J ex-con- 

 jugant in one culture. The same argument applies, with greater 

 force, to the other five lines in Jennings' table, derived from two 

 ex-conjugants which had paired. In other words the variations 

 adopted by Jennings as evidence of different hereditary groups 

 of Paramecium caudatum lie within the variations shown by 

 different lines derived from the same ex-conjugant, and the ar- 

 gument falls completely. 



Now as to the interpretation of tables 1, 2, and 3. It is quite 

 evident that the variations in size have little or no value in he- 

 redity since all came from the same ancestral cell. The varia- 

 tions in the same pure line are often extreme as is the case with 

 line 31 in which there is a difference in mean length of 25.82 mi- 

 crons, between the two measurements, and in mean breadth of 

 9.625 microns, a difference which developed within a period of 

 six weeks, or within about 50 generations by division. Some of 

 the lines on the other hand are more stable, remaining large or 

 small. Thus line 21 remains practically the same in size, while 

 seven other lines differ within less than 10 microns. These may 

 be said to be less variable than the other eight lines. Of the 

 eight lines showing less variability, three are strong conjugating 

 Unes (1, 7 and 21) which have given conjugation epidemics at 

 every test for six m.onths. In the other set, J 8 is the only strong 

 conjugating line, although five of the other lines gave a few pairs 

 each during one of the recent tests (April) . 



On the whole, therefore, it seems probable that Paramecium 

 caudatum may vary in length from about 140 to about 270 mi- 

 crons, the extremes in our measurements. Had we measured all 

 32 lines originally derived from the ancestral ex-conjugant, there 

 is reason to believe that the gaps in our table of means (table 3) 

 would have been filled in, giving a unimodal curve with its mode 

 somewhere in the vicinity of 209 microns. 



