404 MICHIGAN SURVEY, 1908. 



sod, while on larger streams or where the banks are well defined with 

 a deep channel and uniform current, the stick and bank work becomes 

 buried and obscured by the large amount of earth, mud and stones com- 

 posing it. In order to understand the utility of these dams and the 

 resulting ponds, it is necessary to recall the fundamental burrowing 

 character of the beaver, whose burrows and lodges require a submerged 

 entrance, whose winter food must be stored in the bottom of these 

 ponds, the protection thus afforded as a retreat from enemies; and there 

 is yet another important relation which remains to be considered. In 

 a large part of Northeastern North America a marginal zone of floating 

 vegetation, bordered by tamaracks and spruces, tends to line the banks 

 and margins of such streams ponds and lakes as are frequented by 

 beavers. But these conifers are not only unavailable for ifood, but form 

 a. barricade between the water and the hardwoods, aspens, birch, etc. 

 (the food of the beaver) which occupy the higher ground. A further 

 disadvantage of this zone of plant life is that it is very unstable, often 

 even floating, and furnishes no solid ground for burrows, which are 

 the final retreats of the beaver when in danger. Thus the formation 

 of a dam, and the consequent drowning of this unfavorable zone of plant 

 life, tends to bring the water's edge nearer to the hardwoods and solid 

 ground. But to credit all these advantages to the beaver's intelligence 

 is unnecessary because the habit of building dams is of greater geo- 

 graphic extent than these marginal conditions. It seems more probable 

 therefore, that such a habit has proved to be of special advantage under 

 such conditions, rather than that these conditions have developed the 

 habit. 



The beaver meadows are grassland areas, sedges largely, which invade 

 the shallow water and tend to replace the bordering conifers drowned 

 out by the formation of the dams. Such grasslands may be quite exten- 

 sive, and even occupy many acres-, but such results are only secondary 

 products, as far as the beaver.'s needs are concerned, for although the 

 grass stems and roots are eaten to some extent and may be useful in 

 plastering over their houses and in repairing the dams, yet they are 

 apparently not essential features in their economy. 



Geographic Range. — The typical form of this species has a range 

 throughout northeastern North America northward to the tree limit 

 from New Brunswick ; Maine ; New York ; Quebec ; Ontario ; Michigan ; 

 Idaho; Mackenzie (Ft. Simpson) ; Alaska Peninsula and Yukon Valley 

 and Alaska. 



There are three geographic varieties ranging south of the Canadian 

 or typical form ; one in southeastern United States ; another in the Rocky 

 Mountains, and the third on the Pacific coast. Pleistocene beaver 

 remains have been found in New York, Pennsylvania, New Jersey, Vir- 

 ginia, Tennessee, South Carolina, Ontario and Oregon. It is thus seen 

 that for the species as a whole, these fossils do not indicate a range in 

 Pleistocene times materially different from that of the present time. 



The Glacial or post-Glacial extension of range of the Canadian Beaver, 

 from the Atlantic Coast to the Pacific Ocean in Alaska and north to the 

 tree limit, is a r^nge much like that of the Hudson Bay Red Squirrel, 

 and suggests a somewhat similar history. The great development of 

 beavers in this northern region appears closely related to the physical 



