A part of the reservoir plan, however, is that the land above it shall not be cultivated; otherwise the erosion from the tilled fields will promptly fill up the reservoirs, as the present condition of many eastern mill dams so emphatically attests.  The carrying out, therefore, of the Pittsburgh reservoir plan necessitates the exodus of hundreds of thousands of farmers and the restriction of many farming communities to forest or a new type of agriculture.

We cannot spare all this land from tillage.  But fortunately, there are other ways of using it.  Land east of the 100th meridian may be divided into three classes:  First, which in the absence of better estimate covers one third of the area, is hopeless for agriculture because of hills and rocks.  This is mostly now in rather poor forests.  The second class, also covering one third—­by the same estimate—­has been cleared for agriculture, but is so hilly and eroded as to be in a low state of fertility and production.  The third class, the remaining third of the land, is suited to the plow and should be plowed and cultivated much more intensively than it now is.

For the first and second classes of land we need a new type of agriculture, the crop-yielding trees.  Our agriculture, which depends so largely now upon those members of the grass family which we call grains, is the result of accident, not the result of science.  At the dawn of history man had practically all of these small grains, which have probably resulted from the selection and seed saving of the primitive woman, as the race came up from savagery into agriculture.  This primitive woman in selecting plants for her garden and little field, did not pick out the best of nature, or the most productive, or the ultimately most promising; she picked annuals because they gave the quickest return.  And man has left alone and practically unimproved for all these thousands of years nearly all the great engines of nature, the crop-yielding trees, such as the walnut, hickory, pecan, acorn yielding oak, chestnut, beech, pinenut, hazel, honey locust, mesquite, screw bean, carob, mulberry, persimmon, paw-paw, etc., because their slow growth has deterred us from any attempts at improving them.  We have depended upon and greatly improved the quick growing grains, which spend most of their short life in putting up a frame work which promptly perishes; whereas the tree endures like a manufacturing plant.  Further than this, most of the grains have a period of crisis, during which they must receive water or the harvest is almost a failure.  Thus corn must within a short period receive moisture, or it is too late to produce even husks.

Yet trees are the great engines of nature.  The mazzard cherry tree, growing wild throughout the southeastern United States, often yields twenty bushels of fruit.  Fifty bushels and upwards are often obtained from the mature apple trees.  The walnut yields its bushels, the persimmon breaks with fruit.