covered with a vast mer de glâce, causing the environment to be in a state of general instability. The effect of moving masses of ice over the low-lying lands was to smooth their surface, here polishing and striating protruding rocks, there equalizing irregularities by filling up the hollows with the disintegrated materials. The gradual change from a tropical climate to one of Arctic severity, with inter-glacial warm intervals, led to the incoming into Central and Western Europe of different faunas, at one time hailing from sub-tropical and at another from sub-arctic regions. Here, for a time, these immigrants found a congenial home, but ultimately most of them succumbed to the extreme change of climate which subsequently ensued, and the consequent severe struggle for existence to which they were subjected.
To correlate the successive land-areas of the Pleistocene period with the contemporary works of man is a somewhat speculative undertaking in the present state of our knowledge; the most feasible hypothesis is that which makes man's earliest appearance in Britain contemporary with the inter-glacial warm period which immediately followed that of maximum glaciation. As soon as the increasing ice passed its meridian and the environment responded to a warmer temperature, torrential rivers, caused by the melting of the ice, began the work of excavating the river valleys and clearing away the accumulated deposits caused by the previous state of submergence. Con-
