Even more striking than the inverse resemblance of the New World to the Old is the direct similarity of North and South America. In physical form the two continents are astonishingly alike. Not only does each have the typical triangular form which would naturally arise from tetrahedral shrinking of the globe, but there are four other cardinal points of resemblance. First, in the northeast each possesses an area of extremely ancient rocks, the Laurentian highlands 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 series of younger mountains, the two great cordilleras. Finally, the spaces between the three mountain masses are occupied by a series of vast confluent plains which in each case extend from the northern ocean to the southern and bend around the southeastern highlands. These plains are the newest part of America, for many of them have emerged from the sea only in recent geological times. Taken as a whole the resemblance between the two continents is striking. If these four physiographic provinces of North and South America lay in similar latitudes in the respective continents we might expect each pair to have a closely similar effect on life. In fauna, flora, and even in human history they would present broad and important resemblances. As a matter of fact, however, they are as different as can well be imagined. Where North America, is bathed by icy waters full of seals and floating ice South America is bathed by warm seas full of flying-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 South American river, lies sweltering under a tropical sun which burns its grassy plains to bitter dust even as the sharp cold reduced the Mackenzie region to barren tundra. The St. Lawrence flows through fertile grain fields and the homes of an active people of the temperate zone, but the Amazon winds its slow way amid the malarious languor of vast tropical forests in which the trees shut out the sky and the few natives are apathetic with the eternal inertia of the hot, damp tropics.