Being amphibious, they naturally lived by and in water. Their food being tender bark and small twigs of trees, they were forced to gnaw down woody growths to exist, and as these growths near streams usually incline toward the water they naturally fell into or across the channel. Accumulations of driftwood and of the discarded food sticks started dams, and the animal aided the natural construction by adding mud and more sticks. Thus, perhaps, its habits were begun in the remote past by what is called instinct rather than any reasoning quality, yet there remains always the problem as to where instinct stops and reason begins. At any rate there appears to have been no very deep intellectuality about the beaver, notwithstanding its dexterity and ingenuity. It was moulded by the laws of its life exactly as the spider is when it spins a web; yet in the case of the beaver there was a complexity of action that seems extraordinary, although the action apparently was always that which beaver after beaver had employed for an immense period. Where a stream was large and deep or swift, the beaver could not build a dam, nor was it necessary, as it could and did burrow into the banks, excavating a chamber above the water-level, and the primary object of the dam was to supply deep water to cover the lodge entrance. Where waters were continually swift or turbulent and uncontrollable, and especially where they were not bordered by an abundance of cottonwoods, willows, yellow birch, or other favourite food wood, the beaver was absent. For these reasons they were never found in deep canyons. The trappers, as soon as by some bitter experience they discovered this, sought them no further in such localities, hence while these men traversed almost every other foot of the great wilderness, the huge canyons, particularly those of the Colorado River series, were avoided. They continued, therefore, _terra incognita_ long after the remainder of the region was broken; till, in fact, the remarkable boat journey of Major Powell in 1869 fathomed their mysteries. Thus the habits of the beaver controlled widely separated events. [Illustration: Beaver Country. Photograph by J. K. Hillers, U. S. Geol. Survey.] Where brooks or creeks were small with the proper wood growths beaver were abundant, as well as in natural lakes and on all the quieter reaches of the large rivers. Across small streams trees were felled, and with the aid of sticks, mud, and stones the beaver laid up a dam to back up the water and form a basin wherein could be built their lodges with entrances below the surface. On mountain streams these dams one above another often transformed them for long distances into a series of pools and ponds where great numbers of beaver made their homes. In such places a trapper would reap a speedy reward, more particularly as there was no thought of sparing any of the creatures for the future. Often fifty or sixty beaver would be taken in a single night.