This summary statement will convey some idea of the extraordinary phase through which the earth passed in the early part of the present geological era. But it must be added that a singular circumstance prolonged the glacial regime in the northern hemisphere. Modern geologists speak rather of a series of successive ice-sheets than of one definite Ice-Age. Some, indeed, speak of a series of Ice-Ages, but we need not discuss the verbal question. It is now beyond question that the ice-sheet advanced and retreated several times during the Glacial Epoch. The American and some English geologists distinguished six ice-sheets, with five intermediate periods of more temperate climate. The German and many English and French geologists distinguish four sheets and three interglacial epochs. The exact number does not concern us, but the repeated spread of the ice is a point of some importance. The various sheets differed considerably in extent. The wide range of the ice which I have described represents the greatest extension of the glaciation, and probably corresponds to the second or third of the six advances in Dr. Geikie's (and the American) classification. Before we consider the biological effect of this great of refrigeration of the globe, we must endeavour to understand the occurrence itself. Here we enter a world of controversy, but a few suggestions at least may be gathered from the large literature of the subject, which dispel much of the mystery of the Great Ice-Age. It was at one time customary to look out beyond the earth itself for the ultimate causes of this glaciation. Imagine the sheet of ice, which now spreads widely round the North Pole, shifted to another position on the surface of the planet, and you have a simple explanation of the occurrence. In other words, if we suppose that the axis of the earth does not consistently point in one direction--that the great ball does not always present the same average angle in relation to the sun--the poles will not always be where they are at present, and the Pleistocene Ice-Age may represent a time when the north pole was in the latitude of North Europe and North America. This opinion had to be abandoned. We have no trace whatever of such a constant shifting of the polar regions as it supposes, and, especially, we have no trace that the warm zone correspondingly shifted in the Pleistocene.