N. D., Philadelphia, sends a paper on the cause of the absence of trees from our grassy prairies; but there is no need of a further discussion of this subject, as the reason has been amply demonstrated. Whatever can mature seed during a single season, can spread and has spread. Such plants, indeed, formed the vegetation of the grassy prairies. Whatever required to grow up five, ten or fifteen years before producing seeds, could not spread if cut to the ground every year or two, and before producing seeds. A prairie fire would do this. There have been prairie fires, annually, from time ante-dating our history. Trees, therefore, could not seed, and could not spread. They have spread along the streams because water carries the seeds along the streams from unburned districts. This is the sum of a paper by Mr. Meehan in the Proceedings of the Academy of Natural Sciences of Philadelphia, several years ago, and, unless these facts are disputed, there seems no reason to go over the matter at great length again.