This is primarily due to the character of its food. The forest trees themselves furnish what it eats. This means that its food supply is practically limitless. It has very few food rivals. The trunks of full-grown trees offer what is edible to a most narrowly limited number of vertebrates, and therefore--a fact often lost sight of--until man appears on the scene forests do not support anything like the same number and variety of large beasts as open, grassy plains. There are tree-browsing creatures, but these can only get at the young growth; the great majority of beasts prefer prairies or open scrub to thick forest. The open plains of central North America were thronged with big game to a degree that was never true of the vast American forests, whether subarctic, temperate, or tropical. The great game regions of Africa were the endless dry plains of South and East Africa, and not the steaming West African forests. There are, of course, some big mammals that live exclusively on low plants and bushes that only grow in the forest, and some trees at certain seasons yield fruits and nuts which fall to the ground; but, speaking generally, an ordinary full-grown tree of average size yields food only to beasts of exceptional type, of which the most conspicuous in North America are the tree-porcupine and the beaver. Even these eat only the bark; no vertebrate, so far as I know, eats the actual wood of the trunk.
These bark-eaters, therefore, have almost no food rivals, and the forest furnishes them food in limitless quantities. The beaver has developed habits more interesting and extraordinary than those of any other rodent--indeed as interesting as those of any other beast--and its ways of life are such as to enable it to protect itself from its enemies, and to insure itself against failure of food, to a degree very unusual among animals. It is no wonder that, when protected against man, it literally swarms in its native forests. Its dams, houses, and canals are all wonderful, and on the Tourilli they were easily studied. The height at which many of the tree trunks had been severed showed that the cutting must have been done in winter when the snow was deep and crusted. One tree which had not fallen showed a deep spiral groove going twice round the trunk. Evidently the snow had melted faster than the beavers worked; they were never able to make a complete ring, although they had gnawed twice around the tree, and finally the rising temperature beat the teeth, and the task was perforce abandoned.