Later climatic oscillations followed, but ou a decidedly reduced scale. The effect of these was, naturally enough, most marked in northwestern Europe, decreasing gradually southward, and doubtless eventually fading away in the lower latitudes of the continent. It is not necessary for my present purpose to do more than briefly indicate the general character of these later changes so far as they affected our own area.
The local glaciers of the British mountains, some of which, as I have said, actually entered the sea, at last began to retreat. The climate became more genial, and so once more favored the growth of forests, which in many places began to overspread the now dry peat bogs, beneath which the trees of the earlier forest epoch lay entombed. Eventually, however, colder and more humid conditions returned, and small glaciers appeared in a few places among the loftiest heights of the Scottish Highlands. The position of the moraines of these glaciers indicates a height of 3,500 feet for the snow line. The forests now, as before, began to decay in many places, and the bog moss and its allies again extended in all directions, and so, eventually, a second forest bed became entombed in growing peat. It is needless to say that the evidence of these later changes is not restricted to Scotland. The bogs of the two sister countries, and of the corresponding latitudes on the continent, present us with precisely the same phenomena.
The present decayed aspect of the bogs in many places where they formerly flourished, and the fact that certain plants and groups of plants are once more beginning to invade such wastes, shows that we are now living under somewhat milder and less humid conditions.
Although these later climatic oscillations certainly affected the distribution of plants and animals to some extent in northern and north- western Europe, yet the changes brought about were insignificant as compared with those which characterized the alternations of preceding glacial and interglacial epochs. The earlier cold and genial stages were strongly contrasted, and marked by great migrations of flora and fauna. But, as the strange cycle drew to a close, the contrast between glacial and interglacial phases became less and less pronounced and gradually faded away into the present. The steppe fauna vanished from middle Europe during the fourth interglacial epoch, and it never returned. The climatic oscillations that followed were on too small a scale to induce great migrations, and thus the succeeding forest fauna retained its place. Hence in such a section as that seen in the rock shelter of Schweizersbild, we find no recognizable evidence of the climatic changes to which the buried forests and peat bogs and the small local moraines of northern and northwestern Europe bear testimony. It is thus only by correlating and comparing the evidence over the widest area that we are able to get the story completed.
In fine, we have seen that tundras and steppes appeared at successive epochs in prehistoric Europe. The former were contemporaneous with
