The knowledge we gain from a study of the British Tertiary deposits enables us to affirm positively that both the eastern and the northern species arrived in these islands comparatively recently, but that the southern forms must have migrated northward from the Continent long ages ago. Since the northern and the eastern migrations--that is to say, those coming from the north and east--were the last to arrive in Northern Europe, the remains of the animals contained in the most recent deposits of that portion of our continent will furnish us with a clue as to the extent of the area inhabited by them. This is not all, however. It is also possible to discover from these remains the direction which the animals that they belonged to came from. As we shall learn later on, a migration on a vast scale entered Europe during the Pleistocene epoch--the most recent of the geological epochs, during which great extensions of glaciers occurred in the mountainous regions of Europe. The latter period is known to us as the Ice Age or Glacial period. This will be described more fully in Chapter II., meanwhile I may mention that we presume that this migration came from the east, because no remains of the members of that particular fauna are known from Spain, Southern Italy, Scandinavia, Ireland, or from the Balkan peninsula. The number of species evidently belonging to this same migration, moreover, become fewer as we proceed westward, and a large proportion of them still inhabit Northern Asia, though most of them are now extinct in Europe. After having thoroughly studied such a recent geological migration, we learn to understand others better, though the more ancient they are, the fewer are the traces and the more difficult are they to follow. Then again we have to take into consideration the fact, that whilst mammals, particularly the larger herbivores, are forced to migrate frequently owing to scarcity of food or temporary changes of climate, many of the invertebrates remain practically unaffected by either. Most of our land mollusca, for instance, are satisfied with meagre provender, and stand extremes of climate well, as long as there is sufficient moisture. As a result of their peculiar disposition, many of them, no doubt, have survived through several geological epochs, and have witnessed vast geographical revolutions in their immediate surroundings, whilst mammals are comparatively short-lived. Being driven from one country to another, and exposed to innumerable enemies, new types appear and old ones rapidly vanish; in fact, there are almost constant changes in the mammalian fauna as we pass from one epoch to another.