THE FOOD OF FISHES. 



By S. a. FORBES. 



For a clear conception of the general and intricate inter- 

 dependence 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 small- 

 er 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 annihilated it 

 would doubtless be long before the general multitude 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 form- 

 er 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 equi- 

 librium of organic life and activity than in any equal body 

 of land. It forms a little world within itself, — a micro- 

 cosm within which all the elemental forces are at work and 

 the play of life goes on in full, l)ut 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 sensihllity of such an organic complex, — ex- 

 pressed by 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- 



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