92 ORTMANN— THE DESTRUCTION OF [April 23, 



he cannot give positive information as to the presence or absence of 

 particular species of them. But the question of their existence in 

 general in the different streams is easily settled, in fact this is the 

 most conspicuous criterion by which people generally judge the con- 

 dition of a stream — whether there is "good fishing" or not. 



However, the presence of fishes in a stream does not always 

 indicate that the latter is in good shape. The condition of the 

 streams, as we shall see below, often changes during the season ; 

 it is bad in dry weather, but improves when there has been copious 

 precipitation. The fishes are most apt to take advantage of such 

 temporary improvement on account of their great power of locomo- 

 tion (vagility) ; in fact, many fishes migrate more or less regularly 

 up or down stream, and thus may be present at certain seasons in 

 parts of our water-courses, which are barren in other seasons. 



Other vertebrates are of minor importance. Among the mam- 

 mals we should mention the muskrat (Fiber sibethicus). This 

 animal is fairly abundant everywhere, but, as might be expected, 

 tends to disappear, where its food disappears. The latter consists 

 only in part of invertebrates (mussels for instance), while in another 

 part it is vegetable (roots of aquatic plants, and also various parts 

 of land plants). Thus it is understood that the pollution of a 

 stream does not render the existence of muskrats impossible. And 

 further, the bad condition of the water does not harm the animal 

 directly, since it is an air-breathing form. The fact that the musk- 

 rat is decidedly less frequent in polluted streams is probably due to 

 the fact that the pollution is greatest in the vicinity of larger settle- 

 ments, where there is greater danger for them by being hunted 

 by man. 



Of the reptiles, water-snakes {Nairix sipedon and leberis) and 

 turtles should be considered. As regards the former, it is a general 

 rule that they disappear from polluted streams, and very likely not 

 on account of the direct influence of the water upon their body, but 

 on account of the destruction of their food — fish and crawfish. 

 The turtles live in part upon animal, in part upon vegetable food ; 

 they are found, at present, in numbers only in streams which are in 

 good condition, and have disappeared, more or less, in those with 



