INTRODUCTION TO SOCIOLOGY 99
Other natural separations, still more insurmountable than the mountains, are the deserts, such as the Sahara. Sea and desert seem to have the same signification in the ancient languages : mare, "sea," in Latin, in Sanscrit, maru, "desert." In Asia it is necessary to note the steppes, which extend into Europe, and especially the salt desert, which replaced the inland sea around which evolved the primitive tribes of Iran before they were dis- persed in different directions. In India are the jungles which were an obstacle to the formation of national unity and favored the successive conquests of the country by isolating and dividing the particular resisting forces. In America are the pampas, where the wandering habits of the primitive hordes are still preserved.
The forest regions can also, in certain conditions, serve as limits and obstacles. Those of Belgium, Gaul, and Germany presented the same difficulties to the Romans as the American and African forests have in more recent times presented and still present.
After mountains, deserts, forests, seas, and oceans, the rivers, especially the larger ones, are the most natural separations. Like the mountain chains, they form great lines of division, each having its original constitution. One fact is important : often the bed of a river cuts through a mountain chain, whose walls, then become perpendicular to the river bed as, for example, the Rhine, and also, in the upper part of its course, the Meuse. This is a favorable factor in the development of social structure and mass ; in fact, at a certain time the river will become a means of communication between regions previously isolated by the moun- tains which it pierces like the tunnels artificially made later.
Generally, in the upper part of the course, the slope of the river bed is large, the banks high and steep, the width small, and the current more or less violent, according to the height of the mountains where it has its source. In countries like Switzer- land, where several streams have their sources in the mountains, these complications are added to other natural causes of the isolation of local civilizations.
The floods and inundations of rivers and the large marshes
