1026 The American Naturalist. [December, 
The sanitary chemical analysis ordinarily states the amount 
of nitrogen present in four different forms, namely, albuminoid 
ammonia (dissolved and suspended), free ammonia, nitrites 
and nitrates, which represent four stages in the change of or- 
ganic to inorganic matter. Since nitrogen is essential to all 
living matter we naturally expect that organisms will thrive 
best in waters rich in that element. Our statistics show that 
this is the case, and that it is true for each class of organisms 
and for the different conditions of nitrogen tabulated. The 
free ammonia and nitrates appear to be particularly influen- 
tial in determining the amount of life present. For example, 
10 of the 13 ponds low in free ammonia never show maximum 
growths of the Cyanophycee above 100 per c. c., while 4 of the 
7 ponds high in free ammonia commonly have growths above 
1000 per c. c. 
One must be careful in these matters, however, not to mis- 
take cause for effect. Free ammonia, for example, indicates 
organic matter in a state of decay, and instead of representing 
the food of the organisms in question it may represent their 
decomposition. The interaction of the various organisms is a 
very complicated question, and to what. extent one species 
lives upon the products of decay of another is not well known. 
The food supply of the different organisms can only be deter- 
mined by experiments made upon pure cultures, and this sub- 
ject is as broad as the host of the microscopical organisms is 
numberless. For a long time we have been groping in dark- 
ness, but the active interest which is awaking in the study of 
the plankton gives us hope to believe that light has begun to 
awn. 
