Subject to these reservations, when it is clearly understood that when we speak of these main divisions we mean not simple and pure races, but groups of races, then they have a certain convenience in discussion. Over the European and Mediterranean area and western Asia there are, and have been for many thousand years, white peoples, usually called the CAUCASIANS,[72] subdivided into two or three subdivisions, the northern blonds, an alleged intermediate race about which many authorities are doubtful, and the southern dark whites; over eastern Asia and America a second group of races prevails, the MONGOLIANS, generally with yellow skins, straight black hair, and sturdy bodies; over Africa the NEGROES, and in the region of Australia and New Guinea the black, primitive AUSTRALOIDS. These are convenient terms, provided the student bears in mind that they are not exactly defined terms. They represent only the common characteristics of certain main groups of races; they leave out a number of little peoples who belong properly to none of these divisions, and they disregard the perpetual mixing where the main groups overlap. § 3 [Illustration: Negro types] [Illustration: Mongolian Types] Whether the “Caucasian” race is to be divided into two or three main subdivisions depends upon the classificatory value to be attached to certain differences in the skeleton and particularly to the shape of the skull. The student in his further reading will meet with constant references to round-skulled (Brachycephalic) and long-skulled peoples (Dolichocephalic). No skull looked at from above is completely round, but some skulls (the dolichocephalic) are much more oblong than others; when the width of a skull is four-fifths or more of its length from back to front, that skull is called brachycephalic; when the width is less than four-fifths of the length, the skull is dolichocephalic. While some ethnologists regard the difference between brachycephaly and dolichocephaly as a difference of quite primary importance, another school--which the writer must confess has entirely captured his convictions--dismisses this as a mere secondary distinction. It seems probable that the skull shapes of a people may under special circumstances vary in comparatively few generations.[73]