Lastly we come to division No. 3, the Lower Pleistocene, or =Epoch of the Hippopotamus=. The later climate of this age was mild. It came between two glacial periods, owing to the retreat of the glaciers, which had earlier increased in extent so as to produce the second Great Glacial period. The hippopotamus swam in the Thames and Severn in those days, and left its bones and teeth in the older gravels of those and other European rivers, where we now find them. The big almond-shaped and leaf-shaped flint implements of the English (Fig. 73) and French gravels (Fig. 74) belong to this period. We have no knowledge whatever of the men who made them.[8] The mammoth was not there, but another species of elephant (_E. antiquus_) and a peculiar rhinoceros (_R. merckii_). The deepest and oldest deposits in some caves belong to this age, as well as the high-lying gravels of St. Acheuil, of many English river-valleys, and of Chelles on the Seine. This period is not represented by much deposit in caves, though some caves contain very deep-lying layers enclosing bones or teeth of the animals characterising this period. Older than the Age of the Hippopotamus are deposits which are reckoned by geologists as “Pliocene”—no longer Pleistocene—and are called “Tertiary,” not “Quaternary.” The forest bed of Norfolk (regarded by Professor Marcelin Boule as of transitional character, as shown in the tabular view on p. 384 _bis_), the Norwich crag, the Suffolk red and coralline crag, and very extensive sandy deposits all over Europe belong to the Pliocene. The earliest or first great extension of glaciers occurred late in this period. The animals are very different from those of the Pleistocene; the great mastodon and the tapir are there, and the sabre-toothed tiger. Implements manufactured by man are found in the oldest Pleistocene, and there is no reason to doubt that we shall find his workmanship in the Pliocene, too, though it is not admitted that this has yet been done. It is a question still eagerly studied and debated as to whether the roughly chipped flints found in gravels on high downs in the south of England, and called “eoliths,” are (as I think many of them are) the work of man, and whether the high-lying gravels in which they are found are to be regarded as of the oldest Pleistocene Age or as late Pliocene. It is an exciting and deeply interesting field of practical exploration and reasoned inference. [Illustration: FIG. 73.—Flint pick from the Lower Pleistocene of the Thames Valley. Two-thirds the size of the object.] It will have been gathered from what I have said that, in seeking for knowledge of the sequence of events in the period of Palæolithic Man, everything depends upon extreme care in removing the deposits from a cave inch by inch, and keeping all objects found distinct from one another and assigned to their proper layer. The same system is now applied with great success to the exploration of ancient cities in Egypt and Central Asia.