284 
H. S. JENNINGS 
cases save one. In this one a few specimens of i conjugated at 
the same time as k. (In this case the two races did not cross; 
specimens of i conjugated only with other i; those of k only with 
other k.) 
Somewhat similar facts were observed in certain mixtures of 
other races. Thus, in a mixture of C2 + i, made March 2, on 
the next day C2 was conjugating while ^ was not. 
The differences in readiness to conjugate in the experiments 
given above are evidently not due, as might be supposed, to the 
fact that the race which refuses to conjugate has conjugated more 
recently than the other. On the contrary, it is certain that in 
all these cases the race k had conjugated since the other race, yet 
it conjugates anew when the other does not. In some of the cases 
indeed, as we have seen, k goes through two epidemics of con- 
jugation during the course of the experiment, while the members 
of the other race do not conjugate at all, — demonstrating abso- 
lutely that the length of time since the last previous conjugation 
is not what determines the different behavior of the two races. 
The race k is constitutionally disposed to conjugate much more 
readily and frequently than the other races. 
CONJUGATION AMONG PROGENY OF A SINGLE INDIVIDUAL 
Is the greater disposition of k toward conjugation due to the 
fact that this culture was composed originally (as noted above) of 
eight similar pairs, instead of being derived from a single individ- 
ual, as are the other races? Experiment showed clearly that this 
question is to be answered in the negative; conjugation is just as 
frequent when we use from this stock cultures derived entirely 
from a single individual. The experimental facts are as follows: 
A single specimen, one member of a pair that had just conju- 
gated, was isolated from k, November 9, 1908. This was called kb, 
and from it an extensive culture or ^'pure line " was derived. The 
cultures derived from this single ex-conjugant conjugated as 
readily and often as did the original culture k, when treated in 
the same manner. Thus, fresh hay was added to kb February 24, 
