260 LORANDE LOSS WOODRUFF 
vineed, though he cannot prove, that in the final anaylsis the 
paramount factor is food, though many other factors, such as 
excretion products, etc., may play a not unimportant part. Bio- 
metrical study of variation in certain Protozoa shows that the 
average size of the population is smaller after their period of 
ereatest abundance in an infusion and that ‘‘there can be little 
doubt that one of the chief factors which induce saprophytes like 
Chilomonas to disappear from a culture is that the medium no 
longer furnishes proper food (either in amount or kind, or both).’’#8 
Fig. 14 BT group. Hypotrichous fauna at the bottom. 
VIIl. CONJUGATION 
Comparatively few epidemics of conjugation were observed in 
this entire study, and these were chiefly among paramaecia, so 
that the data in this connection are quite meager. It therefore 
has not been possible to make any definite correlations between 
the presence of the phenomenon and the fate of the conjugating 
forms in the infusions. However, a study of the records in regard 
to Paramaecium seems to show that conjugation usually occurs 
when a comparatively large number of individuals are present 
and that immediately following an epidemic there is a temporary 
decline in the number of specimens observed. After this decline 
there may or may not be a large increase in the number of animals. 
It seems clear that in many cases conjugation is coincident with 
sudden changes in the environment. In fact the phenomenon 
may occur in certain cases almost solely among individuals which 
have been carried to the bottom with falling zoogloea. But that 
this does not of necessity bring about conjugation is shown by in- 
fusions Bi and B2. Conjugating paramaecia were not seen at 
18 Pearl: Variation in Chilomonas under favorable and unfavorable conditions, 
Biometrika, vol. 5, 1906-1907. 
