PROTOZOAN FAUNA OF HAY INFUSIONS 257 
in each flask of infusion. This conclusion nevertheless, does not 
necessarily follow from the data, because all of the forms under 
consideration can flourish on a bacterial diet, which, of course, 
was supplied in each case. The interaction of the various forms 
clearly plays a part in the duration of the maximum andthe 
rapidity of the decline. Experiments by the slide method of cul- 
ture, which I have employed in my pedigree culture work, show 
that in culture medium which is the same from day to day prac- 
tically the same ‘sequence’ of maximum numbers occurs and in 
this case it is apparent that chemical changes in the environment 
are not responsible for the results. Further, it is possible to carry 
all the forms under consideration for at least one hundred genera- 
tions by this slide method, and this is sufficiently long to show 
that enough organisms can be produced in a medium which is 
chemically constant to supply, many times over, the number of 
organisms recorded at the maxima.n the regular infusions. Con- 
sequently I think that these observations indicate that the rela- 
tive potential of division of the four forms under discussion is 
adequate, under certain conditions at least, to establish the ob- 
served sequence of maximum numbers, and clearly suggest that 
it may be an important factor in large infusions. 
The data from these infusions lead me to believe that the strictly 
biological factors are of greatest importance, and that it is neces- 
sary to look to somewhat subtle chemical changes in the medium 
for the important chemical factors in the environment. Fine’s 
studies! on these infusions are in accord with this view and indi- 
cate that such general chemical changes-in the environment as, 
for example, titratable acidity are not determining factors, at 
least for these particular species. My work on the excretion prod- 
ucts of Paramaecium shows," however, that such substances have 
an inhibiting influence on the reproduction of this form, and it is 
quite probable that these products affect the sequence, maximum 
numbers, and decline of the various species. In fact Shelford, in 
hisstudies on the ecological succession of fish in ponds, believes that 
1siCf. Hine: loess cits 
16 The effect of excretion products of Paramaecium on its rate of reproduction. 
Jour. Exp. Zool., vol. 10, no. 4, 1911. 
