CONDITIONS FOE CONJUGATION 355 



ing to Zweibaum these supposed racial diversities result from 

 differences in the amount of nutriment. Supposing, however, 

 that acquired diversities may arise in different cultures as a 

 result of differences in environment, these same cultures should, 

 in Zweibaum's opinion, yield conjugants after being subjected 

 to the prolonged experimental procedure which he recommends. 

 But this they fail to do in all cases, as the results of my experi- 

 ments indicate. Cultures which are known to have a common 

 ancestry (various cultures of the same pedigreed stock, descended 

 from one original individual) do respond similarly in these experi- 

 ments. But when races of diverse ancestry are compared they 

 almost invariably respond differently. Two races coming from 

 the same habitat (as from the same pond or stream, aquarium, 

 or hay culture) may sometimes respond similarly under the same 

 conditions, but as a rule they respond differently. Thus, the 

 two races 38 (a and b) and 39a were from the same general source, 

 a pond near Baltimore, but the first underwent two epidemics 

 of conjugation in the laboratory, during which time no conjuga- 

 tion has been observed in 39a. Similarly, the two races 44(^ 

 and 43a were isolated from the same lot of material collected 

 in a river. In 44o^ (and its derived cultures) conjugation has 

 occurred in epidemics of long duration under essentially uniform 

 conditions, whereas in the latter strain {4-3a) no conjugation has 

 been observed since the beginning, under uniform conditions 

 or under the altered conditions of growth which favored conju- 

 gation in 44^ (as when cultures are renewed after dormancy). 

 Su7nmary of observations and experiments on Paramecium 

 caudatum. 1. There are diversities among the various strains 

 of this species as regards their tendency to conjugate under 

 natural and experimental conditions. 



2. Some of these diversities in response under a given set 

 of conditions may be qualitative, as illustrated by the different 

 behavior of two conjugating strains when treated with a partic- 

 ular reagent. That is, the conditions for conjugation in all 

 strains are not identical. 



3. That the several strains in question are racially diverse 

 is supported by the fact that conjugating strains sometimes 



