172 THE POPULAR SCIENCE MONTHLY 



numerous in the same region and there is some Teason 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 mi- 

 grating 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 ques- 

 tion we must consider the physical changes in the bodies of water. 

 Swamps have been drained and their bottoms converted into gardens 

 and cultivated fields. Eiver 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 



