496 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



If any two sets differ in length in the same way at both measure- 

 ments (taken many generations apart) and if the differences between 

 them are each time decidedly greater than the sum of the probable 

 errors of the measurements of the two, then w^e can be assured that 

 we are dealing with really differentiated sets. Now, examination of 

 the extensive series of measurements in Tables X. and XVIII. shows 

 that the probable error of the mean length never reaches two microns, 

 even when the number of specimens is much smaller than in our 

 present measurements, and when conditions are of the most varied 

 character. It is practically certain that the probable error of the 

 mean length would not amount to one micron in any of the compara- 

 tively homogeneous sets with which we are here dealing. If, then, 

 we require a difference of four microns between the mean lengths 

 of the two sets, this difference to have the same sign ( -|- or — ) at 

 both measurements, we shall be within safe limits. Applying this 

 test, we find four lines clearly distinct in the larger or " caudatum " 

 group, while in the smaller or " atirelia " group we can be certain of 

 but two distinct lines (represented best perhaps by / and C2'). We 

 have previously found three distinct lines in the aurelia group {c, g 

 and i, Table XXIV.), so that all together we now have at least seven 

 different lines of Paramecium, showing constant relative differences 

 in length. It is probable that very exact tests would show the dis- 

 tinctness of some other lines of Table XXV. 



The striking difference between adults of different races, under 

 varied conditions, is shown in Fig. 7. Here we have two adults, one 

 belonging to our smallest race (/) ; the other to one of the large races. 



Fig. 7. Extreme adult sizes from different pure lines of Paramecium. 

 a, large individual from a large line, b, small individual from the small line 

 i of Table XXIII., page 488. Both magnified 235 diameters. 



