igos.] JENNINGS— HEREDITY IN PROTOZOA. 407 



really specific distinctness of P. anrcUa and P. caiidatitin on the fact 

 that in one of his pedigree cultures of P. caiidatiiiii the number of 

 micronuclei changed from one to two, remained at two for many 

 generations, and finally changed back again to one. 



The results here published tend to indicate that the distinction 

 into two groups is not without some sort of foundation. But it will 

 be best to reserve the discussion of species until we have more data 

 at hand. We may temporarily speak of the smaller set as the 

 aurcUa group, the larger one as the caudatnin group. In a later 

 part of the paper the question of distinguishing species will be taken 

 up in detail, in the light of full data. 



6. Are Differences in Size Heredit.\ry within E.\ch of the 



Two Groups? 



We have found that among the variations of Paramecium in size 

 are two groups, limited by internal causes, so that even under the 

 same external conditions they differ in size ; these two groups have 

 heretofore been considered two species. But within each of these 

 groups we find likewise many variations in size, so distributed, how- 

 ever, as to produce a curve with a single apex (Diagrams i and 2, 

 etc.). These variations are at times very considerable, as will be 

 evident from an examination of the polygons shown in Diagrams 3 

 and 6 (pages 413. 470 ). or the tables numbered \TI. (page 412) and 

 XX. (page 466). The next question to be considered is: Are the 

 differences in size w'ithin such a group hereditary? That is, do the 

 differences in size depend upon internal conditions, of such a char- 

 acter that the differences will persist in the progeny, even when the 

 external conditions remain the same? 



The experimental answer to this question is to be obtained by 

 isolating individuals of dift'erent size belonging to one of the two 

 groups (either " anrcUa" or " caudatum"), allowing these to mul- 

 tiply and determining whether the progeny show differences in size 

 corresponding to those in the parents. Can we by selection and 

 propagation produce within the limits of a single group races of 

 different mean size?^ 



Experiments designed to answer this question were undertaken 

 in the following way. As representing the caudatum group I 



