pyramid with a triangular base, for when a globe with a fairly rigid surface collapses because of shrinkage, it tends to assume this form. That is what has happened to

Each of the four depressed areas forms a side of our tetrahedron and is occupied by an ocean. The ridges and the areas immediately flanking the oceans form the continents. The side which we may think of as the base contains the Arctic Ocean. The ridges surrounding it are broad and flat. Large parts of them stand above sea-level and form the northern portions of North America, Europe, and Asia. A second side is the Pacific Ocean with the great

s been of almost infinite importance in the evolution not only of plants and animals but of men. The reason is that this arrangement gives rise

mate and Evolution,"

inted out, they cause some forms of life to perish while others survive. Thus climatic variations are among the most powerful factors in causing natural selection and hence in stimulating evolution. Moreover it has lately been shown that variations in temperature are one of the chief causes of organic variation. Morgan and Plough, * for example, have discovered that when a certain fly, called the drosophila, is subjected to extremes of heat or cold, the offspring show an unusually strong tendency to differ from the parents. Hence the climatic variability of the interior of large continents in temperate

ished ma

indigenous life of Africa shows how an equatorial position retards evolution. The still more marked backwardness of Australia with its kangaroos and duck-billed platypuses shows how much greater is the retardation when a continent is also small and isolated. Today, no less than

same general features. Yet their mountains, plains, and coastal indentations are so arranged that what is on the east in

how the chief mountain systems, the Pacific "cordilleras," trend away from one another, southeastward and southwestward. In the centers of the continents they expand into vast plateaus. T

uite different-a mass of volcanic material poured out in the gap where the main chain of mountains breaks down for a space. In neither hemisphere, however, is the main southward sweep of the mountains really lost. In the Old World the cordillera revives in the mountains of Syria and southern Arabia and then runs southward along the whole length of eastern Africa. In America it likewise revives in the mighty Andes, which take their rise fifteen hundred miles east of the broken end of the northern cordillera in Mexico. In the An

lization, like the rivers of water, have flowed from the great cordilleras toward the Atlantic. Half of the world's people, to be sure, are lodged in the relatively small areas known as China and India on the Pacific side of the Old World cordillera. Nevertheless the active streams of civilization have flowed mainly on the other side-the side where man apparently originated. From the earliest times the mountains have served to determine man's chief mig

The aboriginal tribes on the Pacific slope are markedly different from th

hysical and psychical. Virchow has emphasized the fact that the skulls from the northern point of Vancouver's Island reveal an unmistakable analogy to those from the southern coast of California; and this is to a degree true o

the mountains. The eyes are less oblique, the nose flatter, the lips fuller, the chin more pointed, the face wid

issive, and docile; they have less courage, and less of that untamable independenc

"The American R

the north and both become fertile granaries in the center. Before the white man introduced the horse, the ox, and iron ploughs, there prevailed an extraordinary similarity in the habits of the plains Indians from Texas to Alberta. All alike depended on the buffalo; all hunted him in much the same way; all used his skins for tents and robes, his bones for tools, and his horns for utensils. All alike made him the center of their elaborate rituals and dances. Bec

America and the Sahara of Africa. Thence they stretch almost unbroken toward the southern end of the continents. In view of th

American island has only six inhabitants per square mile, while the European country has six hundred. To the east of the British Isles the North Sea, the Baltic, and Lakes Ladoga and Onega correspond in striking fashion to the Gulf of St. Lawrence, the river of the same name, and the Great Lakes from Ontario to Superior. Next the indented shores of western France and the peninsula of Spain resemble our own indented coast and the peninsula of Florida. Here at last the American regions are as favored as the European. Farther south the Mediterranean and Black seas penetrate far into the

nds of Quebec and Labrador in North America and the highlands of Guiana in South America. Second, in the southeast lie highlands of old but not the most ancient rocks stretching from northeast to southwest in the Appalachian region of North America, and in the Brazilian mountains of the southern continent. Third, along the western side of each continent recent crustal movements supplemented by volcanic action on a magnificent scale have given rise to a complex

g-fish and coral reefs. The northern continent is broadest in the cool latitudes that are most favorable for human activity. The southern expands most widely in latitudes whose debilitating monotony of heat and moisture is the worst of handicaps to human progress. The great rivers of the northern continent correspond very closely to those of the southern. The Mackenzie, however, is bound in the rigid bands of winter for eight months each year, while the Orinoco, the corresponding S

a Plata takes its rise in one of the world's most backward plains, the home of uncivilized Indians, heartless rubber adventurers, and the most rapacious of officials. Not infrequently, the degenerate white men of these regions, yielding to the subtle and insidious influence of the tropics, inflict the most outrageous abuses upon the natives, and even kill them on slight provocation. The natives in turn hate their oppressors, and when the chance comes bet