These several stages of the cenozoic were also characterised by great vicissitudes of geography and climate. In the early and middle portions of the eocene, much of the land of the northern hemisphere was under the sea or in the state of swamps and marshes, and there seems to have been a very mild and equable climate, insomuch that plants now limited to warm temperate regions could flourish in Greenland. It is further to be observed that regions such as Mesopotamia, Syria and Egypt, which are known to us historically as among the earliest abodes of man, were at this time under the ocean, as were also rocks that now appear at great elevations in the highest mountains of Europe and Asia. For example, the limestones through which the Nile has cut its valley are marine beds of eocene age, and beds of the same period holding marine remains occur at an elevation of 16,000 feet in the Himalayan region. In the miocene the amount of land was somewhat greater, though large areas of the continents were still under the sea, and the climate was still mild, but for reasons to be stated in the sequel it is not likely that man inhabited the warm continents of this age. The pliocene inaugurates what has been termed a continental period, when the land of the northern hemisphere was higher and more extensive than at present. It was also a time of great physical change, when much erosion of valleys and sculpturing of the surface of the land occurred, and when extensive earth movements and ejections of igneous rock increased the irregularity of the surface and gave greater variety and beauty to the land. The pliocene was altogether a most important period for giving the finishing touches of physical geography, and in it several modern species of land animals were introduced; but we have as yet, as we shall find in the sequel, no certain evidence that man was a witness of the movements and sculpturing of the earth's crust, so important in the preparation of his future home, though statements to this effect have been made on grounds which we shall have to consider.