and the fourth, the English walnut, is a native of Persia and the Caucasus. In times past the distribution was far different. "What information we have is derived only from a confessedly imperfect geological record—a record exposed and explored as yet in the Rocky Mountain region, in Alaska, Greenland, and Central Europe. In all of these places species of either the one or the other genus have been found. It is true that the determination of species or even genera is difficult from fragments of leaves, but, as far as is now known, the following are the facts:
During that period of time known as the Cretaceous epoch, a tropical climate prevailed over the whole of the northern hemisphere, even to the pole itself, and probably also over the whole world. During that time forests of trees flourished over the continent of North America, over Europe, and probably Asia also. These forests were, in many respects, similar in aspect to those which at present clothe portions of the Ohio Valley. What species of trees lived in that part of the country north of Virginia and Tennessee and east of the Mississippi, it is now impossible to tell. But west of that river, and over the larger part of the Rocky Mountain region, there grew forests in which poplars, willows, oaks, beeches, sycamores, gums, magnolias, and perhaps walnuts and hickories, were the prevailing types. The remains of all but the last two have been found in the immense series of Cretaceous rocks of the "Western States and Territories, having fortunately been preserved in the deposits of the shallow seas or great lakes which then occupied that part of the American Continent. The genera mentioned are but a few of those of which remains have been found, but they indicate a similarity in the flora then to that of the present epoch. Many of the same genera are found in strata, of presumably the same age, in Alaska, Greenland, and parts of Europe, and these facts indicate not only a similar climate in all these localities, but a similar forest aspect.
In the Tertiary formations, coming after the Cretaceous, the resemblance to modern trees is still more striking. During this later period, that part of the Rocky Mountain region which had before been under water was elevated above the surface, but immense basins remained which were filled with brackish or fresh water. It is in these fresh-water deposits that the abundant remains of ancient forests are found. Many of the same types present in the Cretaceous period are found in the Tertiary in still greater numbers. If there be any doubt about the occurrence of walnuts and hickories in the former period, there can be no question about their being in the latter one. The number of species of oaks, poplars, figs, ashes, magnolias, and many others equally well known, was increased. But we shall here consider the two genera, the walnut and the hickory.
In Europe one species of walnut is found in the Cretaceous rocks, and a doubtful one is mentioned as found in rocks of the same age in
