THE BEAVERS OF NORTH AMERICA 73 



enlarge the dam, making it both longer and higher, 

 so that a greater area is flooded. There comes a 

 time, however, when a limit is reached, either 

 through the natural conformation of the land or 

 through the beaver's inability to build a dam beyond 

 a certain height. They are then confronted with 

 the problem of getting to and from the trees, on 

 which they depend for food, without the necessity 

 of travelling on land. By a very gradual method, 

 which probably originated by the enlarging of small 

 natural openings in the bank, the beaver worked 

 out the canal scheme. When this happened no 

 one can say, as the ditches very soon lose all trace 

 of their origin, but there is every reason to believe 

 that it has been going on for very many thousands 

 of years. Where conditions are favourable canals 

 reach the extraordinary length of upwards of 1,000 

 feet. Of course, this gigantic work is not all 

 done at once, but gradually, as the supply of 

 food trees goes back further and further from the 

 pond. It might be presumed that these canals are 

 only run through level country, but here is the 

 greatest evidence of the engineering ability of the 

 beaver : they build their canals uphill by means of 

 weirs or dams, the distance between them varying 

 according to the gradient. Yet they never work 

 uphill unless it is made absolutely necessary by the 

 contour of the land. They avoid hills just as care- 

 fully as man does when engaged on similar work, 

 but when confronted with an unavoidable obstacle 



