vary action with reference to changing needs is the best possible index of the mental power of animals. Guided by the exhibition that has been given us by the beavers, we are justified in considering them to be the one group of mammals which has gained a distinct, rational constructive power. This feature makes them decidedly the most interesting group for investigations which may be expected to throw light on the problems of animal intelligence. From the economic point of view the species has a certain importance for the reason that it affords one of the most valuable kinds of fur that has ever been marketed. The domestication of the beavers to the point where they would tolerate the presence of man should not, provided they could be protected against the depredations of poachers, be a matter of any difficulty. The colonies of these animals require only what is afforded by vast realms of our wildernesses--flowing streams of moderate fall with timber upon their banks. They are not particular as to the species, so that swift-growing kinds of trees such as the poplars may be made to serve their needs. The natural growth on a hundred acres of otherwise worthless land would probably be sufficient to maintain a colony of average size containing say twenty-five individuals. In the region about the great lakes and for some distance to the northward and to the east and west there are great areas amounting in the aggregate to some hundred thousand square miles that would apparently be well suited to the nurture of this form, and which in the present condition of the country, as well as for the immediate future, cannot be turned to better use. It may be remarked that the domestication of the beavers would afford yet another means, in addition to those above noted, whereby we might be able to win some profit from the great wilderness of the north, which is, so far as our existing means of appropriating its resources, of little use to mankind. The only evident way by which we may hope to win profit from this part of our continent is by using it as a field for rearing animals that have yet to be subjugated; none of our captive varieties are fit for the service.