558 H. S. Jennings and George T. Hargitt. 



microns. Conjugates much less readily than k. Rate of fission 

 greater than in k. More than three years in the laboratory. 

 (See Jennings, '08, for extensive studies of growth, environmental 

 action and selection in this race.) 



^. The smallest race studied; length about 90 microns, under 

 conditions in which c has a length of about 100 microns. Con- 

 jugates less readily than k. Rate of fission as in c, more rapid' 

 than in k. Two years and six months in the laboratory. (See 

 Jennings, '08, for many measurements of i, in comparison with 

 g, and for studies of the effect of selection in this race.) 



A word may here be added regarding the distribution of the 

 various races. There appears to be a general impression that 

 animals answering to the description of aurelia are rare as com- 

 pared with caudatum; Calkins ('06) urged this as one argument 

 against the distinctness of P. aurelia. My experience has not 

 confirmed this impression. The small forms belonging to P. 

 aurelia are not readily visible to the naked eye, and are not con- 

 spicuous even with the microscope, while the large individuals 

 of P. caudatum are very conspicuous; it is natural, therefore, that 

 the former do not attract attention so readily as the latter. I 

 believe that this is the reason why caudatum has been supposed 

 more common. My impression from long continued work with 

 the diverse races is that aurelia is as frequent as caudatum. 

 Many samples of ''wild" material, from ponds, drains, etc., con- 

 tain races of both aurelia and caudatum. Others contain aurelia 

 only; others caudatum only. Apparently samples containing 

 aurelia occur as often as those containing caudatum. 



1 1 . Precautions necessary for work with pure races 



A final word should be said regarding methods of work in 

 studying these races. Demonstration of the constancy and uni- 

 formity of the races can of course come only when the most rigid 

 care is taken to prevent contamination and admixture. By 

 accidental transference of individuals of one race to cultures of 

 another (which may happen with the greatest ease), it will of 

 course be easy to get striking cases of apparent splitting of one 



