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AMD @ Osa NOUN tate iezey 
BY S. A. FORBES. 
For a clear conception of the general and intricate interdepend- 
ence of the different forms of organic life upon the earth, one can 
not do better than to study thoroughly the life of a permanent body 
of fresh water,—a river or smaller stream, or, better than these, a 
lake. The animals of such a body of water are, as a whole, curi- 
ously isolated,—closely related among themselves in all their inter- 
ests, but so far independent of the life of the land about them that, 
if every terrestrial plant and animal were annibilated, it would doubt- 
less be long before the general multitude of the inhabitants of the 
lake or stream would feel the effects of this event in any very impor- 
tant way. 
Further, the greater difficulty of communication between the dif- 
ferent parts of a water system as compared with the different regions 
of the land, is such that the former are much the more sharply 
limited. There is very much less interchange of all kinds between 
two branches of the same stream, for example, than between the 
tracts of land which they separate. Consequently, one finds in a 
single body of water a far more complete and independent equilibrium 
of organic life and activity than in any equal body of land. It forms 
a little world within itself,—a microcosm within which all the ele- 
mental forces are at work and the play of life goes on in full, but 
on so small a scale as to bring it easily within the mental grasp. — 
Nowhere can one see more clearly illustrated what may be called 
the sensibility of such an organic complex,—expressed by the fact 
that whatever affects any species belonging to it, must speedily have 
its influence of some sort upon the whole assemblage. He will thus 
be made to see the impossibility of studying, any form successfully 
out of relation to the other forms,—the necessity for taking a com- 
prehensive survey of the whole as a condition to a satisfactory 
understanding of any part. If one wishes to become acquainted 
with the black bass, for example, he will learn but little if he limits 
himself to that’ species. He must evidently study also the species 
upon which it depends for its existences, and the various conditions 
upon which these depend. He must likewise study the species with 
which it comes in competition, and the entire system of conditions 
affecting their prosperity. Leaving out any of these, he is like one 
who undertakes to make out the construction of a watch, but over- 
*From Bull. No. 3, Ill. State Lab. Nat. Hist., November, 1880. : 
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