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From the highlands of central Mexico across the entire United States and northward into Canada extends the great mid-continental prairie. On the east it is in contact with the deciduous forest. From this central mass the prairie extends westward beyond the Rocky Mountains across Wyoming into eastern Utah, and southwestward through northern New Mexico in to northern Arizona. The Palouse Grasslands occurred in eastern Oregon and Washington, and other parts of the far Northwest. The Pacific Prairie was found in the Great Valley of California, and the-Desert Plains Grassland still occupies much of southern Arizona and New Mexico and southwestern Texas. Together they constitute the Grassland or Prairie Formation, which is the most extensive and most varied of all the natural units of vegetation of the North American continent. In fact, native grassland formerly covered 38 per cent of the entire land surface of the United States.
Throughout the entire Prairie Formation the climate is more favorable to grasses than to trees or shrubs or, indeed, to any other type of vegetation. But within the vast range of grassland climate, marked differences exist in degrees of favorableness for growth. These are illustrated especially by differences in precipitation and relative rates of evaporation. Temperature and length of growing season are of less importance, for all the grasslands seem to lie well within summer temperature limits favorable to growth of the gtass life-form. Since rainfall decreases and evaporation increases from east to west in the great mid-continental area, there-have resulted several different kinds or associations of prairie, each limited in extent by a distinctly different minor grassland climate.