DESTRUCTION OF AQUATIC ANIMALS. 109 
Man has hitherto hardly anywhere produced such climatic 
or other changes as would suffice of themselves totally to banish 
the wild inhabitants of the dry land, and the disappearance of 
the native birds and quadrupeds from particular localities is to 
be ascribed quite as much to his direct persecutions as to the 
want of forest shelter, of appropriate food, or of other condi- 
tions indispensable to their existence. But almost all the pro 
cesses of agriculture, and of mechanical and chemical industry, 
are fatally destructive to aquatic animals within reach of their 
influence. When, in consequence of clearing the woods, the 
changes already described as thereby produced in the beds and 
currents of rivers, are in progress, the spawning grounds of fish 
are exposed from year to year to a succession of mechanical dis- 
turbances; the temperature of the water is higher in summer, 
colder in winter, than when it was shaded and protected by 
wood ; the smaller organisms, which formed the sustenance of 
the young fry, disappear or are reduced in numbers, and new 
enemies are added to the old foes that preyed upon them; the 
increased turbidness of the water in the annual inundations 
chokes the fish; and, finally, the quickened velocity of its 
current sweeps them down into the larger rivers or into the 
sea, before they are yet strong enough to support so great a 
change of circumstances.* Industrial operations are not less 
alone to exhaustion by fishing, for in the waters of the valleys and flanks of 
the Alps, which have been inhabited and fished ten times as long by a denser 
population, fish are still very abundant, and they thrive and multiply under 
circumstances where no American species could live at all. On the southern 
slope of those mountains, trout are caught in great numbers, in the swift 
streams which rush from the glaciers, and where the water is of icy coldness, 
and so turbid with particles of fine-ground rock, that you cannot see an inch 
below the surface. The glacier streams of Switzerland, however, are less 
abundant in fish. 
* A fact mentioned by Schubert—and which in its causes and many of its 
results corresponds almost precisely with those connected with the escape of 
Barton Pond in Vermont, so well known to geological students—is important, 
as showing that the diminution of the fish in rivers exposed to inundations is 
chiefly to be ascribed to the mechanical action of the current, and not mainly, 
as some haye supposed, to changes of temperature occasioned by clearing. 
