EFFECT OF CONJUGATION 303 



matters connected with conju!2;ation. It is easy to induce 

 epidemics of pairing at intervals of about a month, and they some- 

 times occur at much shorter inter\^als. In order to make clear 

 the conditions with which we are dealing, it is necessary to give 

 a brief account of the history of this race. 



As a pure strain, the race k is derived from a single ex-conju- 

 gant isolated November 9, 1908. This individual itself came 

 from a culture derived from eight pairs of conjugants taken 

 February 4, 1908, these eight pairs being themselves derived 

 from 10 single individuals of similar size, taken from a wild 

 culture January 29, 1908. Thus even before the destruction 

 of all but this single individual of November 9, the race k was 

 derived from few individuals, which very possibly all came from 

 one. Our first experiment given below (Experiment 4) made use 

 of this race k before its absolutely certain derivation from a 

 single individual; all the rest employed k as known to be a pure 

 strain. 



Epidemics of conjugation were observed in this race eight 

 times between January 29 and its absolute purification on Novem- 

 ber 9. Since November 9 a great number of conjugations have 

 been observed; records have been kept of at least twenty. 



That portion of the race still in existence (July, 1912), and the 

 part on which some of the chief experiments were performed, has 

 descended from many successive conjugations, in which all the 

 surviving progeny were derived from a single member of a pair. 

 Eight such conjugations have been observed, so that all the 

 animals now existing (and used in Experiments 13 and 14, below) 

 are derived from eight generations of the strictest inbreeding — ■ 

 all the members of each of these generations being derived from 

 the fission of a single individual. This inbred race seems healthy 

 and vigorous, so long as cultivated in mass culture. But it 

 appears to have lost the ability, which it had at the beginning, to 

 propagate for any considerable period on shdes. On this account 

 it has of late become unavailable for comparative work on such 

 questions as the rate of reproduction and the way this is affected 

 by conjugation or by other conditions; a fact which has caused 

 much trouble and loss of time. JNIany extensive experiments 



