It will be apparent that a colony of beavers would soon exhaust the supply of trees bearing edible bark within reach of the shore of their stream, unless they had some means of reach ing new and more distant supplies. In truth, where the banks are steep, this soon happens, and the beavers must then seek a new place. Where the forest is low and level, however, they will excavate canals which are gradually extended farther and farther into the woods on each side of the pond, and so enable themselves to reach more and more fresh trees. In some of the swampy forests about the headwaters of the Mississippi, which was perhaps the head quarters of beaver life in this country, these canals have been known to extend several hun dred feet, and in such places colonies of beavers have maintained an existence of more than 200 years. These channels are kept free from weeds and of a proper depth; and the most important service which the dam renders is to maintain the right level of water in these canals, so that they may always be used as the avenues of the industrious conununity.
The American beaver seems to have carried its architectural work to a higher degree of perfection than the European beaver was ever known to do, although in Siberia, where simi lar climatic conditions prevail and it is neces sary for them to erect houses impervious to the great cold and to the attacks of marauding animals, they . come near to equalling their American cousins. There is little record of such
structures being made primitively in central Europe, and the beavers now living in the streams of Germany and Austria make few at tempts at either dams or houses but are con tent to dwell in their bank-burrows.
The beaver thrives in confinement and there are colonies in the zoological gardens of the larger cities. In 1913 for the first time beavers bred 'in their pond in the New York Zoological Park.
The substance called castoreum is obtained from two glandular pouches in the beaver, closely connected with the organs of reproduc tion, and probably of service in attracting the sexes to one another in the rutting season. It is a secretion having a powerful, peculiar, pun gent odor and was formerly in demand for medical purposes. At present its only use is as a scent-bait for traps. Fossil remains of beavers have been found as far back as the middle of the Tertiary period. Fossils of small sized species with some distinctive peculiarities occur in the Miocene rocks of the western United States; and a huge beaver. therium) existed in Europe in the Pliocene age. Consult Harting, 'British Animals Extinct within Historic Times' (London 1880) ; Mar tin, H. T., (Castorologia' (Montreal 1892) ; Mills, E. A., (In Beaver World' (Boston 1913) ; Morgan, (The American Beaver and his Works' (Philadelphia 1868) ; Ingersoll, 'Life of Mammals) (New York 1907) ; Seton, (Northern Mammals) (New York 1909).