10 
is possible to us to unravel and to elucidate in general and in 
detail the system of aquatic life in a considerable district of 
interior North America. 
So vast a subject must of course be intelligently divided 
and studied part by part, in some systematic order, to avoid a 
dissipation of effort and to insure the speedy attainment of some 
definite and tangible results. Its most obvious divisions are the 
systematic, the biographical, and the oecological; and this is 
the order, broadly speaking, in which the general investigation 
must be carried on. Both systematic and biographical biology 
have a high independent value in our scheme, but both are with 
us chiefly means to the remoter end of a study of the inter¬ 
actions of associate aquatic organisms, and of their relations to 
nature at large. It is thus the oecological idea which is to lead 
in the organization and development of our work. A systematic 
survey of the biological assemblage is a necessary preliminary 
step, and the tracing of life histories and the recognition and 
description of immature stages is a scarcely less essential pre¬ 
requisite ; for without the knowledge which these studies are to 
give us, it would be obviously impossible to make any compre¬ 
hensive study of variations, distribution, and oecological rela¬ 
tionships. 
The oecology of the Illinois River is greatly complicated, 
and the difficulty of its study intensified, by certain highly and 
irregularly variable elements of the environment. Apart from 
those secular and more or less inconstant features of climate 
and weather which must be taken into account wherever such 
studies are prosecuted, we often have here the evidently very 
large and highly intricate reactions produced by periodic varia¬ 
tions in the river level, and the consequent enormous extensions 
and corresponding diminutions of the mass of the waters and of 
the area covered by them. Fortunately for the possibilities of 
success in so difficult a field, progress in it does not require that 
the entire system of life should be studied as a unit at first. 
Special problems may be selected, of a kind to be brought easily 
within the available time and the capacities of the individual 
investigator, which, being worked out one by one, may be later 
brought together as contributions to a solution of the larger 
problems involved. While it is true, for example, that nothing 
in or about the waters studied which in any notable wav affects 
t/ */ 
