The Great Central Plain of North America produces more than one-quarter of the world's wheat, and about four-fifths of the corn. The southern part of the great Arctic plain, and its extension, the plains of the Baltic also yield immense quantities of grain and cattle products. The coast-plains of the Atlantic Ocean, on both the American and the European side, are highly productive. River flood-plains are almost always densely peopled because of their productivity. The bottom-lands of the Mississippi and the Yangtze Rivers are among the chief food-producing regions of the world. Lacustrine plains, the beds of former lakes, are also highly productive regions. The valley of the Red River of the North is an example, and its wheat is of a very high quality. Fertile coast-plains and lowlands that are adjacent to good harbors, as a rule are the most thickly peopled regions of the world. In many such regions the density of population exceeds two hundred or more per square mile. The reason is obvious. Life seeks that environment which yields the greatest amount of nutrition with the least expenditure of energy. The study of a good relief map shows that, as a rule, the Pacific Ocean is bordered by a rugged highland, which has a more or less abrupt slope, and a narrow coast-plain. Indeed, the latter is absent for the greater part. The slopes of the Atlantic, on the other hand, are long and gentle--being a thousand miles or more in width throughout the greater part of their extent. The area of productive land is correspondingly great, and the character of the surface features is such that intercommunication is easy. [Illustration: A RIVER FLOOD-PLAIN--A REGION ADAPTED TO CULTIVATION] The result of these conditions is evident. The Atlantic slopes, though not everywhere the most densely peopled areas, contain the great centres of the world's activities and economies. In the past 400 years they have not only overtaken the Pacific coast races, but have far surpassed them. They are now entering upon a commercial invasion of the Pacific nations that is resulting in a reorganization of the entire industrial world. =Topography and Trade Routes.=--As the settlement and commerce of a country grow, roads succeed trails, and trails are apt to follow the paths of migrating animals. Until the time of the Civil War in the United States, most of the great highways of the country were the direct descendants of "buffalo roads," as they were formerly called. In the crossing of divides from one river-valley to another, the mountain-sections of the railways for the greater part follow the trails of the bison. This is especially marked in the Pennsylvania, the Baltimore and Ohio, and the Chesapeake and Ohio railways; in some instances the tunnels through ranges have been constructed directly under the trails. The reason is obvious; the instinct of the bison led him along routes having the minimum of grade.