In the preceding statements we have spoken of the Beaver as forming a single species; but it has long been a moot question with zoologists whether the Beavers of the Old and New Worlds were or were not specifically identical. The external differences are very slight, and those observed in the skull, upon which most stress has been laid, do not appear to be of sufficient importance for the separation of the animals as distinct species. They consist chiefly in the greater breadth of the anterior portion of the skull, including the inter-orbital space; the extension farther back of the nasal bones, the greater size and depth of the basilar cavity and the more anterior position of the auditory bullæ, in the European Beaver; but the examination of large series of specimens has proved that the skulls from both hemispheres present many exceptions, in which one or more of the peculiarities which they ought to exhibit do not occur: a circumstance which of necessity greatly invalidates the distinction founded upon such characters. Dr. Ely sums up the results of an extended investigation in the following words:--“The extremes of difference, in their aggregate, on the one side and on the other, are sufficiently striking to justify us in regarding them as varieties of one and the same species; while the want of constancy in these peculiarities suggests the inference that these variations are due to long separation of the races, and to accidental causes, rather than to original diversity of the stock.” The Beaver may thus be regarded as a species with two geographical forms (varieties or sub-species), viz., _Castor fiber_, var. _europæus_, and _Castor fiber_, var. _canadensis_. So much has been written upon the habits of the Beaver, that the following short statement will suffice to give the leading facts in the natural history of the animal, the accounts of the marvellous sagacity of which, given by the older writers, have, perhaps, invested it with an exaggerated interest. In populous countries the Beaver is contented, like the Otter, with a long burrow for his residence; but in the wilder regions of Siberia and North America his dwelling-place is a much more complicated affair. But even in these regions, according to some authorities, a certain number of Beavers--always males--show a lazy unwillingness to take part in the common labours of the colony, and these, as idlers, are expelled from the community, often with rather severe treatment, and then take up their abode by themselves in holes, which they dig out in the banks of rivers, whence they are called “terriers.” On the other hand, it would appear that the building instinct which is so remarkably manifested by the Beaver is not always extinct even in those which inhabit populous countries, for we have a most interesting account from M. Meyerinck of the construction of a lodge, and even of a dam, by the colony of Beavers on a tributary of the Elbe.