CONDITIONS FOR CONJUGATION 367 



Summary of observations and experiments on Paramecium 

 aurelia. 1. In this species, as in the preceding, there are 

 diversities in respect to conjugation. Some strains show a 

 marked tendency to conjugate repeatedly at regular intervals, 

 while others, under the same uniform conditions, do not conjugate 

 during long periods of time. Diversities in response to the same 

 set of experimental conditions are sometimes indicated, as 

 when one race shows a greater tendency to conjugate in a particu- 

 lar saline medium than in an aqueous medium, another race 

 so treated showing a lesser tendency. 



2. These diversities probably have a racial basis, for if two 

 cultures of the same strain, each having had a similar history, 

 be subjected to the same treatment they show about an equal 

 tendency to conjugate. Furthermore, when two different strains 

 are compared they are found to preserve about the same relative 

 degree of susceptibility, under the given set of conditions, 

 as they showed in the beginning. 



Comparing, now, these two species of Paramecium as regards 

 conjugation, we find certain points of resemblance as well as 

 differences. They each respond, in general, to the same set of 

 experimental conditions — dormancy, renewed division, etc. — and 

 show some similarity in their behavior when treated with salts. 

 In each species there is a tendency for conjugation to occur 

 periodically in some strains, although the intervals between 

 periods are not of like duration. In the smaller species (P. 

 aurelia) the life processes appear to be contracted into a shorter 

 space of time than in the larger, but the same kind of internal 

 factors seem to be operative in both species. 



The role which these internal factors play is best shown when 

 we subject a strain to a uniform environment, or by testing 

 experimentally (using the same experiment each time) at frequent 

 intervals during ,a period of several months. Studied in this 

 way, the organisms show periods of greatest susceptibility 

 and refractory intervals during which conjugation does not 

 occur. 



