402 JENNINGS— HEREDITY IN PROTOZOA. [Aprih^, 



this way, but it was thought wiser to be conservative in this matter. Thus 

 the space between two perpendicular lines of the polygons includes three 

 of the groups of the correlation table, and is marked at its base with the 

 middle value of the three groups which it includes. 



In making the polygons for breadth, it was found that there was little 

 evidence of error due to unconscious preference for certain numbers in 

 making this measurement. This is probably due to the comparatively small 

 numbers of units in the breadth measurement, and to the fact that it is 

 possible to hold both limits of the measurement on the scale sharply in the 

 eye at once, while this is hardly possible in measuring length. In the poly- 

 gons for breadth, therefore, one unit of the polygorL was made to correspond 

 to one unit of measurement (four microns). 



In all the polygons the numbers to the left indicate percentages of the 

 entire number, so that all the polygons are of equal area, whatever the 

 number of specimens on which they are based. The only exception to this 

 is in the case of the double polygons a and A, of Diagram i, resulting from 

 plotting the random sample of Table I. Since this sample falls into two 

 groups, the entire (double) polygon was made of twice the area of the 

 other polygons. Each half polygon therefore becomes approximately equal 

 to any one of the single polygons of the other diagrams, thus permitting 

 ready comparison. 



The numbers at the foot of the diagrams are the dimensions in microns. 

 Each number corresponds to the value of the center of the column beneath 

 W'hicli it stands. 



5. Two Groups of Paramecia. 



Tims the Paramecia in otir nattiral culture I fall into two groups 

 which are almost completely .separated, so far as length is concerned, 

 but which overlap a certain amount in breadth. Characteristic out- 

 lines of varied members of the two groups, drawn to the same scale, 

 are shown in Fig. i. 



Are these two groups permanent differentiations, such as might 

 be called distinct species, or are the differences possibly due merely 

 to temporary dimorphism of some sort? To answer this question 

 individuals of the two sizes were isolated and allowed to multiply 

 separately, in cultures made of boiled hay. After varying periods 

 of time 100 individuals, taken at random, were measured from each 

 of these pure cultures, and the frequency polygon derived from the.se 

 was compared with the two (nearly distinct) polygons from the 

 original culture. The following cultures were made and measured : 



I. Fifty of the smaller individuals were selected from the orig- 

 inal culture, placed together, and allowed to multiply for twelve days 

 (from April 10 to April 22). The measurements of 100 of this 



