CONJUGATION IN PARAMECIUM 
281 
from a single individual, I have kepc in the laboratory for periods 
varying from one aad a half to three years. Some of these races 
have apparently never conjugated during the whole period. Others 
have conjugated once, — in some cases as an epidemic, in others 
only scattering individuals conjugating. Other races have con- 
jugated two or three times, while certain races have conjugated 
many times, epidemics of conjugation occurring at brief intervals. 
Observations on these races were not continuous, so that con- 
jugation may have occurred at times when it was not observed. 
No stress is laid, therefore, upon the fact that certain races were 
not seen to conjugate at all. But the very great difference between 
the different races in the readiness to conjugate is clear, and is con- 
firmed, as we shall see later, by precise experimentation. Certain 
races could be depended upon with certainty to r>,onjugate when- 
ever new food was added to the culture, while in other cases this 
never occurred. Some of the facts are as follows: 
The large race D, used for the chief part of my study of inheri- 
tance and variation in size (see Jennings, 1909), was derived from 
a single iudividual taken April 10, 1907; it has now been in the 
laboratory, unmixed, for two years and ten months. During 
the first year ic was under practically daily observation, and deter- 
mined attempts were made to induce conjugation. It has never 
been observed to conjugate. 
Now contrast the case of D with that of the culture k. This 
was not a pure line, but was derived from eight small pairs of 
conjugants of equal size, taken January 29, 1908. (As we shall 
see later, the fact that eight pairs were taken, instead of a single 
individual, does not affect the tendency to conjugate.) The 
culture k consisted of rather small individuals, having but two 
micronuclei. It was not observed so closely as D, but inci- 
dentally epidemics of conjugation were observed in it on the 
following dates: (1908) January 29 ; February 4 ; February 17-20; 
February 26; September 12; October 19; November 9; (1909) 
February 24-27; March 1-6; March 13; March 26; April 14; 
April 19; May 11 ; May 24; June 13 ; June 19; October 7-10; (1910) 
January 29; February 24. 
