One of the first questions meeting us is that which relates to the point in geological time signalised by the advent of our species. In the Eocene period our continents were being gradually raised out of the ocean, and were still in great part under the waters, which several times returned upon the land, and seemed ready again to engulf it. In this period not only have we no traces of man, but all the higher animals of that age are now extinct. In the later Eocene and Miocene the extent of land became greater, but it was so disposed as to allow the influx into the Arctic Sea of vast volumes of heated water from the equatorial regions; and there may have been also astronomical causes at work to increase this influx of warm water, and so to raise the temperature of the Arctic regions still higher.[83] The middle period of the Tertiary was undoubtedly a time very favourable to the wide distribution of the higher forms of life both animal and vegetable. But we cannot trace man or any of the contemporary mammals back to the Miocene. In the Pliocene the continents had attained to their present elevations, and climates were not dissimilar from those prevailing at present; but still we have no certain indication of the presence of man; and if other modern mammals extend back to this period their number is very small. In this age also the greater part of the continents must have been covered with a great thickness of soil and disintegrated rock favourable to vegetation, and there seemed nothing to preclude the introduction of man. But a new and at first sight most unfavourable change was to intervene. Whether through internal changes affecting the distribution of land and water, or through astronomical vicissitudes, the northern hemisphere, and possibly the whole world, entered on an era of refrigeration, the so-called “Glacial Age” of the Post-Pliocene or Pleistocene period. That in this period our continents as far south as the latitude of 40° were overwhelmed with ice or ice-laden seas is rendered evident by the fact that the whole surface up to several thousands of feet above the sea-level has been bared of its accumulated _débris_ and polished and grooved by ice, and laden with boulders and other glacial deposits, while in many places at heights of even 1,000 or 1,200 feet these deposits contain sea-shells of species now living in the colder parts of the ocean. These phenomena do not exist in the tropical regions, except in the vicinity of high mountains, but they recur in the southern hemisphere. It is still uncertain whether the period of greatest cold in the two hemispheres was at the same time or in successive ages. Geologically, however, they are approximately contemporaneous, both occurring between the end of the Pliocene and the modern period; but nevertheless they may not have coincided in absolute date.