I am disposed to think that more bogs in the Northern States owe their origin to beavers than to accidental obstructions of rivulets by wind-fallen or naturally decayed trees; for there are few swamps in those States, at the outlets of which we may not, by careful search, find the remains of a beaver dam. The beaver sometimes inhabits natural lakelets and even large rivers like the Upper Mississippi, when the current is not too rapid, but he prefers to owe his pond to his own ingenuity and toil. The reservoir once constructed, its inhabitants rapidly multiply so long as the trees, and the harvests of pond lilies and other aquatic plants on which this quadruped feeds in winter, suffice for the supply of the growing population. But the extension of the water causes the death of the neighboring trees, and the annual growth of those which could be reached by canals and floated to the pond soon becomes insufficient for the wants of the community, and the beaver metropolis now sends out expeditions of discovery and colonization. The pond gradually fills up, by the operation of the same causes as when it owes its existence to an accidental obstruction, and when, at last, the original settlement is converted into a bog by the usual processes of vegetable life, the remaining inhabitants abandon it and build on some virgin brooklet a new city of the waters. [Footnote: I find confirmation of my own observations on this point (published in 1863) in the North-West Passage by Land of Milton and Cheadle, London, 1865. These travellers observed "a long chain of marshes formed by the damming up of a stream which had now ceased to exist," Chap. X. In Chap. XII, they state that "nearly every stream between the Pembina and the Athabasca--except the large river McLeod--appeared to have been destroyed by the agency of the beaver," and they question whether the vast extent of swampy ground in that region "has not been brought to this condition by the work of beavers who have thus destroyed, by their own labor, the streams necessary to their own existence." But even here nature provides a remedy, for when the process of "consolidation" referred to in treating of bogs in the first chapter shall have been completed, and the forest re-established upon the marshes, the water now diffused through them will be collected in the lower or more yielding portions, cut new channels for their flow, become running brooks, and thus restore the ancient aspect of the surface.