444 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
to invalidate the conclusions that the European and the American Beaver 
constitute different species. 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 wart 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. It is conceded by the advocates of a diversity 
of species that the Beavers of the Old and the New World cannot be dis- 
tinguished by any external characteristic. The same is true of their habits 
and instincts, except so far as they have been evidently controlled by external 
influences. The castoreum secretion is variable, even in the European 
Beavers, and there are facts to show that the elements of the food of the 
animal are found in it.* The differences observed in it, being more of degree 
than of kind, are not of such a character as to render it improbable that they 
are due to the influence of climate, food, and accidental causes.”+ 
A careful analysis of the above-noted cranial differences between the 
European and American Beavers shows that they consist mainly in (1) the 
greater general breadth of the anterior portion of the skull, resulting in a 
greater interorbital breadth, wider nasal bones, wider muzzle, and consequently 
wider incisors ; (2) the relatively greater posterior extension of the nasals ; 
(3) the greater size and depth of the basilar cavity ; and (4) in less marked 
and rather more inconstant features of difference ina few other points. Con- 
ceding with Dr. Ely their varietal or subspecific distinctness, the two forms 
may be thus conveniently diagnosed : 
Castor fiber var. fiber.—Dorsal surface of the interorbital region gener- 
ally as broad as, or broader than, long; nasals extending backward beyond the , 
posterior border of the anterior orbital process; basilar cavity deep and large, 
bullee placed more anteriorly, ete. 
Castor fiber var. canadensis.—Dorsal surface of the interorbital region 
generally longer than broad ; nasals generally not reaching beyond the middle 
of the anterior orbital process; basilar cavity comparatively shallow, etc. 
Synonymy and Nomenclature.—In respect to the distinctive name of the 
American form, that of canadensis of Kuhl evidently has priority ; the amert- 
*The castoreum of the American Beaver is well known to differ very materially from that of the 
Old World Beaver, and has a very much smaller commercial value. Chemical analyses show that the 
castoreum of the Russian Beaver contgins more volatile oil, castorin, and resin, and much less curbonate 
of lime, than that of the American Beaver. 
t Morgan’s “ The Beaver and his Works”, p. 299. 
