THE CAUSES OF VEGETATIONAL CYCLES

The Demonstration of Vegetational Cycles.—The work of the past decade has shown most clearly that there are cycles of vege tation which are comparable to cycles of erosion; in each there is a period of youth, which is characterized by vigor of development and by rapidity of change ; in each there is a period of old age, which is characterized by slowness of transformation and by approach to sta bility, or at least to equilibrium. At the close of the vegetational cycle there is no such universal feature as the ultimate plain of the physiographer, since the final stage varies with the climate, and hence is called a climatic formation1. In the eastern United States, the final formation is a mesophytic deciduous forest*; farther to the north and in the Pacific states, it is a coniferous forest; in the great belt from Texas to Saskatchewan, the final formation is a prairie; and in the arid southwest, it is a desert. In every case, the ultimate plant formation is the most mesophytic which the climate is able to support in the region taken as a whole. In a prairie climate there may be trees, but they occur for the most part near lakes or streams, or in