344 HOYT S. HOPKINS 



For our purpose, then, three general questions should be kept 

 in mind: 1) Do there exist differences between various races 

 of Paramecium with respect to conjugation? 2) If so, what are 

 they? 3) From a comparative study of different races of the 

 same species and of the different species, what can be said in 

 general regarding the nature of the conditions for conjugation 

 in the Infusoria? 



Any answer to the third question must depend, in part at least, 

 upon the facts brought out in connection with the other two. If 

 constant natural differences in the time interval between suc- 

 cessive epidemics of conjugation in different races and species 

 can be demonstrated, it would coincide better with the view that 

 conjugation is primarily a function of some internal factors, 

 although, of course, it would not exclude the setting in operation 

 of these factors by environmental conditions. The complete 

 subordination of conjugation to direct environmental conditions 

 might lead us to regard the process as mainly environmental in 

 its origin, and perhaps directly adaptive in its functional 

 significance. 



MATERIAL AND METHODS 



Twenty-four races of Paramecium, each derived originally 

 from a distinct individual, were used in the course of this work; 

 eleven representing P. caudatum, and thirteen, P. aurelia. 

 Many other races were isolated, but owing to unfavorable culture 

 conditions died out before reaching the 'mature' condition of 

 old cultures. 



This dying out of unbalanced cultures is conditioned largely 

 by the racial characteristics of the protozoan isolated, it being 

 ill adapted to the conditions of growth supplied in the laboratory. 

 This is shown by the fact that other races which were reared with 

 difficulty to the condition of stable cultures in the beginning 

 were found to show, some months later, this same inability 

 to develop rapidly when isolated individuals were reared again 

 under rich culture conditions. Deleterious strains of bacteria 

 undoubtedly serve as the immediate cause of the death of a 

 culture in most cases, but the diversities between races of pro- 



