364 HOYT S. HOPKINS 



few months of its history, usually under fairly uniform conditions; 

 whereas later the process could be evoked only after renewing 

 cultures which had undergone dormancy. The; natural perio- 

 dicity which characterized the early stages became suppressed, 

 and the relative number of conjugants obtained likewise fell 

 off considerably as time progressed. The effect is probably 

 not due to unfavorable culture conditions long maintained, 

 although this may play some part, nor to close inbreeding; but 

 is probably connected in some way with the adjustment of the 

 organisms to a state of continuous, rapid multiplication. This 

 point will be treated more at length in part 2 of this paper (p. 371). 



The eight races designated 13a, 14a, loa, 16a, and 23a, 24a, 

 25a, and 26a were derived on February 28, 1919, from the same 

 source — an abandoned hay culture containing various infusoria. 

 Each was descended from a single exconjugant, the original 

 individuals of 13a and 23a having come from the two members 

 of one pair, UfO, and 240. from another pair, etc. It is important 

 to compare these races from the start, since all had been sub- 

 jected to the same environmental influences previously. The 

 cultures were examined frequently for eight weeks, while nu- 

 merous tests for conjugation were made with dense watch-glass 

 cultures isolated from the main jars. All the stock cultures 

 developed rapidly, except 24ci and 26a, in which growth was more 

 tardy, owing in part to overnutrition, to some extent possibly 

 to racial weaknesses. Conjugation occurred in two cultures, 

 23a and 25a, during the period of continuous cultivation, but 

 not in the six others. 



In 23a several pairs were observed as early as March 11th, 

 eleven days after the previous union, and again on March 17th. 

 They were found in larger numbers (5 to 15 per cent) from March 

 18th to 22nd, but only casually thereafter until March 31st, 

 when conjugation practically ceased in the continuous culture. 

 Isolation experiments gave as high as 40 per cent on March 15th, 

 and subsequently until April, in diminishing numbers (March 

 18th, 40 per cent; March 20th, 30 per cent; March 26th, 5 per 

 cent, etc.). 



