514 JENNINGS— HEREDITY IN PROTOZOA. [April 24. 



the distinction that has been made between two species, the larger 

 ones representing the supposed species caudatum, the smaller ones 

 aurelia. But a line or race was found with mean length lying mid- 

 way between these groups, at about 150 to 160 microns. 



The smaller or aurelia lines were found to be, under the same 

 conditions, as a rule a little broader in proportion to the length than 

 the larger or caudatum lines. But the difference is slight and the 

 two sets overlap extensively in this matter; slight differences in 

 environment quite obscure the difference in proportions. 



The differences among the different lines were found not to be 

 due to different periods of the life cycle. By beginning with con- 

 jugating pairs of different sizes, distinct pure lines were as readily 

 isolated as by beginning anywhere else in the cycle. 



6. After becoming thoroughly familiar with diff'erences due to 

 growth, to environment, and to divergent ancestry, a further attempt 

 was made to change by selection the characteristics of pure lines, or 

 to break such lines into strains of differing size. In spite of much 

 work directed on this point, it was found that selection within a pure 

 line was quite without effect. Large individuals of the line produce 

 progeny of the same mean size as do the small individuals. To this 

 matter we return in later paragraphs. 



2. Determining Factors for Dimensions, Variations and 

 Correlations. 

 Based on the analysis of the factors in variation above set forth, 

 a summary can be given of the various determining causes of the 

 different dimensions, the proportions, the amount of variation and 

 the correlations observed in samples of different cultures of Para- 

 mecium. We may take as an example such a sample as is shown in 

 Table LXI. (appendix) from a " wild " culture. 



I. The various different lengths depend upon the following factors : 

 (a) The collection embraces a number of different races or lines, 

 having different lengths even when all conditions are the same. We 

 have seen that different lengths varying from less than 100 to more 

 than 200 microns may be included as a result of this fact. The 

 mean length may not represent any of these races (this is the case in 

 Table L). 



