RODENTIA 275 



the growth of young trout until they are large enough to do well in 

 the swift water of mountain streams. However, it is asserted that 

 harmful gases or poisons develop from the decomposition of organic 

 matter in the stagnant ponds, not only affecting the fish in the ponds, 

 but also getting into streams and affecting them. This probably de- 

 pends somewhat upon the temperature of the water and the amount of 

 organic matter in the water, and both of these would in turn be gov- 

 erned to some extent by altitude and latitude. Therefore, beaver ponds 

 are probably often quite beneficial along cool, high mountain streams, 

 but may be quite detrimental under some circumstances in warmer 

 localities. Furthermore, a newly-made pond may be much less favor- 

 able to fish than an old, stabilized pond. It is a well-known fact that 

 many beaver streams are excellent trout streams, and often good fish- 

 ing is found at the beaver dams. This subject is exceedingly complex, 

 and information now at hand is not sufficient to warrant very positive 

 statements concerning the problem. 27 



Beavers are accused of destroying browse which would be useful 

 to deer, but as beavers are rather localized, while deer move about 

 freely, this accusation does not seem to be very serious. Furthermore, 

 in some localities beaver meadows are favorite feeding grounds for deer 

 when not too much snow covers the vegetation. 28 



FAMILY CRICETIDAE NATIVE AMERICAN RATS, MICE, ETC. 



Though some of the wild mice are very destructive, many of them 

 are more or less useful. Some of them are much more highly insectiv- 

 orous than is generally supposed. In Michigan, of 2611 cocoons of 

 the destructive larch sawfly on one tract, 62.5 percent had been opened 

 by mice in order to eat the larvae, 4.6 per cent had been parasitized by 

 insect enemies, and 0.51 per cent had been fungicized; of 1908 cocoons 

 on another tract, 66.3 per cent had been opened by mice, 3.5 per cent 

 parasitized, 0.2 per cent fungicized ; of 408 cocoons, 80 per cent had 



County, New York, ibid., pp. 1-41, 1922. Kendall, The status of fish culture in our 

 inland public waters, and the role of investigation in the maintenance of fish re- 

 sources, Roosevelt Wild Life Bull., n, 205-351, 1924; A trout survey of Alleghany 

 State Park in 1922, ibid., iv, 286-491, 1927. Knight, Losses in trout fry after dis- 

 tribution, Science, LXII, 590-591, 1925; LXIII, 209-210, 1926; Losses in speckled trout 

 after distribution, Science, LXV, 525-526, 1927. Lawrie, Beaver vs. trout, Fins, Feath- 

 ers and Fur (Bull. Minnesota Game and Fish Dept.), No. 27, p. 5, 1910. Beakbane, 

 The case against the beaver, Forest and Stream, LXIX, 203, 236, 239-240, 1922. 



21 See Johnson, The beaver in the Adirondacks: Its economics and natural his- 

 tory, Roosevelt Wild Life Bull., iv, 501-541, 1927. Bailey, Beaver habits and experi- 

 ments in beaver culture, U. S. Dept. Agric., Tech. Bull., No. 21, 1927. 



28 Johnson, Roosevelt Wild Life Bull, rv, 569-572, 587, 1927. 



