296 
H. S. JENJ^INGS 
are frequently presented as pools evaporate in summer, — and at 
such times, as we have seen above, oDservation shows that con- 
jugation actually occurs. In all these cases conjugation takes 
place immediately after a period of rapid multiplication, when 
the nutritive conditions begin to decline — just as we have before 
set forth to be the case in the laboratory-bred cultures. 
In strains that conjugate only very rarel}^, it is naturally diffi- 
cult to study precisely the external conditions favorable to con- 
jugation, and in strains that are never observed to conjugate, such 
as D, it is of course impossible. In one strain that was observed 
to conjugate but twice in nearly three years (the race c), the ani- 
mals appeared at conjugation to be in a different condition from 
that observed in k. They were very thin, so as to appear starved. 
Apparently, the conjugation in this race comes on at a later stage 
in the disappearance of nutrition than is the case with k. But 
conjugation could by no means be induced at will by starvation 
in this race c, so that, with but two epidemics of conjugation, 
both appearing unexpectedly, it is difficult to judge as to the re- 
quired conditions. It seems probable that the external conditions 
favorable to conjugation are in c somewhat different from those 
in k. 
FACTORS DETERMINING CONJUGATION IN PARAMECIUM 
Putting all the facts together, it is clear that the occurrence of 
conjugation (hke most other functions of animals) depends partly 
on internal conditions, inherited from parent to progeny, partly 
upon external conditions. If we compare two races such as k 
and L2, we find that under the same conditions (and even when 
the two have lived for many generations under the same condi- 
tions), one race conjugates w^hile the other does not. The differ- 
ence is therefore due evidently to internal factors, to constitutional 
differences between the two races, inherited from parent to prog- 
eny. On the other hand, if we compare two cultures from the 
same race k, under different conditions, we find that one culture 
conjugates but the other does not. In this case the difference 
in behavior is evidently due to difference in external conditions; 
