298 
H. S. JENNINGS 
sary cyclical degeneration, remedied by conjugation, becomes still 
greater. But this question is wrapped up so closely with the 
problem of the effects of conjugation on the life history .that it 
will be best to reserve our discussion of it for a later paper dealing 
with experiments on the subject just mentioned. 
SUMMARY 
1. The conditions determining conjugation differ greatly in 
different races of Paramecium (aurelia or caudatum). Some races 
conjugate frequently, and under conditions readily supplied in 
experimentation. Others, under the same conditions, conju- 
gate very rarely or not at all. 
2. The interval between conjugations may be very short, as in 
k, where epidemics of conjugation occurred, under proper condi- 
tions, at intervals of from two weeks to a month; in one case the 
interval between successive conjugations was but five days. 
In other races conjugation occurred only at intervals of a year or 
more. In one race Z), though watched with care, conjugation 
has never been observed in a period of three years in the labo- 
ratory. 
3. Frequent re-conjugation may occur among the progeny of 
a single individual. In k conjugation has been observed to occur 
in a certain direct line five times in succession, — both conjugants 
in each pair being descendants of a single member of the preced- 
ing pair. The descendants of an ex-conjugant reconjugated in 
one case after but four divisions. 
4. Conjugation occurred, in the races favorable for experimenta- 
tion, not as a result of starvation, but at the beginning of a de- 
cline in the nutritive conditions, after a period of exceptional 
richness that has induced rapid multiplication. At the time of 
conjugation the animals are often in good condition, and mul- 
tiplication may still be in progress. 
In the races conjugating less readily, the external conditions 
favoring conjugation are probably somewhat diverse from those 
just set forth, yet of a similar general character. 
