In the course of the pliocene the previously high temperature of the northern hemisphere was sensibly lowered, and at its close the pleistocene period introduced a cold and wintry climate, along with gradual and unequal subsidence of the land, the whole producing that most dismal of the geological ages, known as the 'glacial period.' At this time much of the lower land of the continents was submerged and the mountains became covered with snow and ice, leaving space for vegetable and animal life only toward the south and in a few favoured spots in the higher latitudes. There is much difference of opinion among geologists as to the extent, duration and vicissitudes of this reign of ice, but there can be no doubt that it destroyed much of the animal and vegetable life of the pliocene, or obliged it to migrate to the southward. In this period great deposits of mud, sand and gravel were laid down, which prepared the world for a new departure in the succeeding age. This we may name the post-glacial, or early modern period, and in it we have the most certain evidence of the existence of man, though the geographical arrangement of our continents and their animal inhabitants were in many respects different from what they now are. If geologists are right in the conclusion already stated, that the close of the glacial period is as recent as 7,000 years ago, this will give us a narrow limit in time for the age of man, at least under his present conditions. While, however, there is an absolute consensus of opinion among geologists as to the existence of man at or about the close of the glacial age, in the northern temperate regions at least, there are some facts which have been supposed to indicate a pre-glacial human period, or the advent of man even as early as the middle of the cenozoic time. These merit a short consideration. CHAPTER III THE EARLIEST TRACES OF MAN