Page:Popular Science Monthly Volume 83.djvu/176

172 numerous in the same region and there is some reason to think that certain fishes are spreading eastward across Illinois and Indiana, the border states of the prairie region. Just how general this eastward migration may be among the various classes we do not know, but a reason for it is not difficult to find. Clearing the forests has brought about conditions somewhat similar to those of the prairies, and the small species that can exist in the pastures, meadows and roadsides now find congenial surroundings farther east, and in the east competition is less severe than it was formerly because the forest fauna has diminished.

In this connection it is worthy of note that there is also a slight but rather general tendency of our fauna to migrate northward. This may be the latter end of a general northward migration begun some thousands of years ago when the great ice sheet that then covered most of northeastern North America began to retreat. There were few, if any, animals in the region at that time, but, as the ice melted, and the climate became warmer, the region was again occupied by a fauna migrating into it from the south. At present this migration is not rapid enough to be of much importance.

In my brief enumeration above I mentioned several species that seem to do equally well in wooded or treeless regions. These are the species that are fitted par excellence to survive, and, barring some that are ill adapted because of special modifications, they are the ones that are holding and will continue to hold their own in point of numbers.

Animals inhabiting fresh water are beavers, muskrats, ducks, geese, snipe, frogs, fishes, mussels, crayfishes and a host of other animals, small in size but numberless in individuals.

What is the tendency among these animals? To answer this question we must consider the physical changes in the bodies of water. Swamps have been drained and their bottoms converted into gardens and cultivated fields. River courses are straightened and the waters confined within their banks. Sewage and refuse dumped into streams pollute their waters, and sometimes wipe out the fauna completely, and always injure the larger species. Forests are cleared away, with the result that streams, once dotted with placid pools, now become raging torrents at one season and dry channels at another.

Such changes can not fail to have a disastrous effect on all classes of aquatic animals. The diminution of waterfowl, food and game fishes, muskrat and beaver, which is the result, is too well known to need comment; the decrease of small animals is almost as great.

It may be argued that the work of drainage is counterbalanced by the digging of canals and the building of reservoirs for irrigation. There is no question but that building great reservoirs in arid regions will somewhat increase the aquatic fauna of the surrounding districts. But the isolation of these bodies of water and the obstructions in their