Topographic and climatic differences led, however, to lines of minor ethnic variation, which, for environmental reasons, cross each other at right angles. Topographically considered, the western hemisphere combines longitudinal uniformity with latitudinal diversity, so that in this respect the two sections of the continent are very much alike. Both North and South America have their mountain ranges along the west and the mural masses of each section are succeeded by deforested plains and forested river valleys extending to the Atlantic coast. As a result, there is a corresponding variation of ethnic types running through both continents from west to east, showing more or less marked distinctions between the men of the mountain, the men of the plain, the men of the forest and the men of the shore. Owing to its extension across almost all the degrees of latitude, the western hemisphere offers, on the other hand, an extreme longitudinal range of climate, so that in this respect there is a striking difference between the two Americas. Both triangular sections have their bases on the north and their apexes turned towards the south in such a way that the northern continent is mostly temperate and the southern continent predominantly tropical. As a result, there are likewise lines of ethnic variation running along the longitudes, which distinguish the inhabitants of the northern or temperate continent from the inhabitants of the southern or tropical continent. From the fact that they cross each other at right angles, however, these longitudinal and latitudinal variations tend to neutralize each other to a large extent and leave a relatively uniform type. It is possible on this account, despite the diversity that exists among the American aborigines, to distinguish the Eed man from his human fellows by his brown or copper-colored skin; his lank black hair, which is nearly round in cross section; his deep-set beady black eyes; his aquiline nose; his massive jaws; and his finely formed figure.
There has been no attempt in the foregoing, either to make an exact analysis of the ethnic make-up of mankind, or to follow the process of varietal differentiation in detail. The sole purpose of the enquiry has been, by combining the analytical and geographical methods of investigation, to indicate the probable order of racial ramification and to determine in a general way under what environmental conditions the ethnic differentiation of mankind occurred.
