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 adam 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 
