CONJUGATION IN PARAMECIUM 
297 
the inherited tendency is the same in the two sets. What char- 
acterizes a given race then is an inherited method of responding 
to the environment, — this differing in the different races. 
But even in the same race, the response to the same environ- 
mental conditions is not always the same. Under the same ex- 
ternal conditions sometimes conjugates; at other times it does 
not. What are the internal differences that decide, within a given 
race, whether conjugation shall occur or not? In the case of k it is 
possible to form a more or less definite idea of what the determin- 
ing internal conditions are. If the animals are thin and starved, 
they do not conjugate. If they are becoming well-fed, and in- 
creasing in vitality, they do not conjugate. If they have been 
well-fed. but have now begun to decrease, then they do conjugate. 
The differences between specimens that will, and others that will 
not, conjugate are thus more or less evident to the eye. But in 
such races as L2, c and D, where conjugation occurs only rarely, 
even though they are repeatedly put through the cycle of nutri- 
tive changes just described, the internal changes that initiate 
conjugation are much more difficult to define. 
Is the change which brings on conjugation in the nature of 
decrepitude due to age; is it a consequence of the gradual run- 
ning down of the machinery of growth and multiplication, worn 
out through a '^cycle'^ of non-sexual reproduction, as is held by 
the commonly received theory? When we consider that in such 
strains as k conjugation may be repeated at intervals of two weeks 
or a month, or in some cases with an interval of but five days, and 
with only four generations between the two conjugations — it be- 
comes difficult to believe that this hypothesis furnishes the cor- 
rect explanation. A cycle" in k might consist of but four gen- 
erations! And when we recall further that W^oodruff (1909) has 
kept Paramecium multiplying by fission for 1238 generations 
(26 months) without conjugation and without any indication of 
senile degeneration, and that Enriques (1907) has kept other in- 
fusoria for many hundreds of generations in the same way; when 
we recall also that I have kept the progeny of a single individual 
isolated from all others for three years without any indication of 
degeneration, doubt of the correctness of the theory of neces- 
