Various theories and speculations have been formed as to the origin of the prairies.  One of them, is, that the forests which formerly occupied these plains were swept away at some remote period by fire; and that the annual fires set by the Indians have continued this state of things.  Another theory is, that the violent winds which sweep over them have prevented the growth of trees; a third, that want of rain forbids their growth; a fourth, that the agency of water has produced the effect; and lastly, a learned professor at the last meeting of the Scientific Convention put forth his theory, which was, that the real cause of the absence of trees from the prairies is the mechanical condition of the soil, which is, he thinks, too fine,—­a coarse, rocky soil being, in his estimation, a necessary condition of the growth of trees.

Most of these theories seem to be inconsistent with the plain facts of the case.  First, we know that these prairies existed in their present condition when the first white man visited them, two hundred years ago; and also that similar treeless plains exist in South America and Central Africa, and have so existed ever since those countries were known.  We are told by travellers in those regions, that the natives have the same custom of annually burning the dry grass and herbage for the same reason that our Indians did it, and that the early white settlers kept up the custom,—­namely, to promote the growth of young and tender feed for the wild animals which the former hunted and the cattle which the latter live by grazing.

Another fact, well known to all settlers in the prairie, is, that it is only necessary to keep out the fires by fences or ditches, and a thick growth of trees will spring up on the prairies.  Many fine groves now exist all over Illinois, where nothing grew twenty years ago but the wild grasses and weeds; and we have it on record, that locust-seed, sown on the prairie near Quincy, in four years produced trees with a diameter of trunk of four to six inches, and in seven years had become large enough for posts and rails.  So with fruit-trees, which nowhere flourish with more strength and vigor than in this soil,—­too much so, indeed, since they are apt to run to wood rather than fruit.  Moreover, the soil in the groves and on the river bottoms, where trees naturally grow, is the same, chemically and mechanically, as that of the open prairie; the same winds sweep over both, and the same rain falls upon both; so that it would seem that the absence of trees cannot be attributed wholly to fire, water, wind, or soil, but is owing to a combination of two or more of those agencies.

But from whatever cause the prairies originated, they have no doubt been perpetuated by the fires which annually sweep over their surface.  Where the soil is too wet to sustain a heavy growth of grass, there is no prairie.  Timber is found along the streams, almost invariably,—­and, where the banks are high and dry, will usually be found on the east bank of those streams whose course is north and south.  This is caused by the fact that the prevailing winds are from the west, and bring the fire with them till it reaches the stream, which forms a barrier and protects the vegetation on the other side.