THE FOOD OF FISHES. 
By S. A. FORBES. 
For a clear conception of the general and intricate in 
terdependence of the different forms of organic life upon 
the earth, one cannot 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, curiously isolated 
— closely related among themselves in all their interests, 
but so far independent of the life of the land about them 
that if every terrestrial plant and animal were annihi- 
lated it would doubtless be long before the general multi- 
tude of the inhabitants of the lake or stream would feel 
the effects of this event in any very important way. 
Further, the greater difficulty of communication be- 
tween the different 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 independ- 
ent 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 elemental 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 — ex- 
pressed Hy the fact that whatever affects any species be- 
longing 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 
comprehensive survey of the whole as a condition to a sat-, 
isfactory understanding of any part. If one wishes to be- 
