which, passing the southern mountain-boundary of this rainless district, peopled China and the regions between it and India—thrusting the aborigines of these areas into the hilly tracts; and which has sent successive waves of invaders not into these regions only, but, from time to time, into the West. We have the Aryan race, overspreading India and making its way westward through Europe. We have the Semitic race, becoming dominant through North Africa, and, spurred on by Mohammedan fanaticism, conquering parts of Europe. That is to say, besides the Egyptian race, which, seeming by its alliances to have originally been of low type, became powerful in the hot and dry valley of the Nile, we have three races, widely unlike in type, and speaking languages classed as fundamentally distinct, which, from different parts of the rainless district, have spread as invaders over regions relatively humid.
Original superiority of type was not the common trait of these races: the Tartar type is inferior, as well as the Egyptian. But the common trait, as proved by subjugation of other races, was energy. And when we see that this common trait, in races otherwise unlike, had for its concomitant their long-continued subjection to these special climatic conditions—when we find further, that from the region characterized by these conditions, the earlier waves of conquering emigrants, losing in moister countries their ancestral energy, were overrun by later waves of the same races, or of other races coming from this region, we get strong reason for inferring a relation between constitutional vigor and the presence of an air which, by its warmth and dryness, facilitates the vital actions.
A striking verification is at hand. On turning to the rain-map, it will be seen that, of the entire New World, the largest of the parts distinguished by the absence of shade as almost rainless, is that Central American and Mexican region in which indigenous civilizations developed; and that the only other rainless district is that which formed part of the ancient Peruvian territory—the part, moreover, in which the pre-Inca civilization has left its most conspicuous traces. Inductively, then, the evidence justifies in a remarkable manner the physiological deduction.
Nor are there wanting minor verifications. Comparisons among African races are suggestive of similar contrasts in constitution, similarly caused. Of the varieties of negroes Livingstone remarks ("Missionary Travels," p. 78): "Heat alone does not produce blackness of skin, but heat with moisture seems to insure the deepest hue;" and Schweinfurth, in his lately-issued "Heart of Africa," similarly remarks on the relative blackness of the Denka and other tribes living on the alluvial plains, and contrasts them with "the less swarthy and more robust races who inhabit the rocky hills of the interior" (vol. i., p. 148). There seem, generally recognizable, corresponding differences in energy and social advance. But I note this difference of color arising
