What we--in our ignorance of the inner life of those creatures who have always shared the rich heritage of this world with ourselves--term instinct, has taught the beavers to provide against drought, and to keep up a certain necessary depth of water, by making a dam right across these smaller rivers just at what they know to be a convenient distance from their houses. The manner in which they construct this depends on the locality where they live. If the current is not strong--if there is only a slight motion of the water--the dam is made almost straight; but in proportion as the stream is a rapid one, the dam must be more curved, presenting its convex side towards the current. Where beavers have been allowed to build for a long period undisturbed, their dams become in time, through the persistent industry with which they repair them, a bank so solid that it resists quite a strong on-rush of water or even of ice. Vegetation plants itself on this--willows, birches, and poplar-trees take root. Sometimes so large a thicket is formed that birds build there, and the whole makes a charming colony of happy and busy life. The dams are built in some rivers of trees which are often five or six inches in diameter. These the beaver cuts down with his wonderful sharp incisor teeth.
In lakes and ponds also the beavers have their habitat. They like much the narrow creeks which so often connect the lakes of North America. The currents help them to convey the wood and other materials to their dwellings. A certain depth of water is, of course, necessary for their purpose. Driftwood is utilised by them in building, as well as the green boughs of willows, birches, and poplars. But mud and stones are used also, welded all firmly together, and the different parts of the dam must, of course, be of equal strength. In the same manner, that is, of the same materials, they construct their dwellings, but they are not built with equal care; their construction is rougher than that of the dams. The only thing essential in the work is that they should be made watertight, so that they may have dry sleeping-quarters. Sometimes a house is just big enough for one family, but larger dwellings are also made, such as will house a great number of animals. When this is the case, each family has its own apartment, with a separate door communicating only with the water, never with the home of any other family. The wood is laid crosswise, nearly horizontal, leaving a cavity in the centre. The smaller branches, that project uselessly, are cut off with the teeth, and they are thrown in with the rest to form a good safeguard against any falling in of the mud through the roof.