TROUT WATERS AND CARP WATERS 



If, through their gills, all fishes breathe the oxygen 

 dissolved in the water which surrounds them, their 

 individual requirements differ. Some, which have an 

 active respiration, must have around them a consider- 

 able proportion of oxygen. This is the case with the 

 trout. Others, more modest in their demands, are 

 content with a smaller proportion. So it is with the 

 carp family. Now, fresh waters, which differ con- 

 siderably in their content of dissolved oxygen, vary 

 considerably from place to place and from season to 

 season. Their mixing with the air; their purity; the 

 vegetation on their banks, have obviously a good deal 

 to do with it, but temperature is the most important 

 factor, for according to its height the rate of solution of 

 oxygen changes; the colder the water, the greater its 

 power to dissolve oxygen. Warm water is definitely 

 poor in oxygen. The carp flourish in it; the trout, on 

 the other hand, cannot get on in it at all, unless it is 

 sufficiently stirred by the air: and, in cold water, swift 

 flowing, rich in dissolved oxygen, the trout get on very 

 well and the carp not at all. 



Each species has a capacity and faculties of its own. 

 These, being appropriate to its needs, cause it to settle 

 down in those places where it is best able to satisfy 

 those needs. There it settles naturally; there it takes 

 up its abode, is ready to live, and disinclined to leave. 

 The trout do not go down to the rivers of the plain, 

 where the water is warm in summer and flows slowly. 

 Similarly the carp and tench make no attempt to reach 

 the mountain torrents even if they get the chance. 

 Each in its suitable environment realizes in that 

 environment, in the close relation of its constituents, 

 an organic well-being, in which its functions are able 

 to find their due fulfilment and equilibrium. Outside 

 certain limits this equilibrium is broken; the state 

 of well-being disappears, and the organism declines. 

 The living creature can maintain itself and prosper 

 only when it finds around it, in its environment, the 

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