Pleistocene formations which occur outside of the glaciated areas—the valley-drifts, lignites and peat, lacustrine deposits, and calcareous tufas—tell the same tale of changing climatic conditions. During one stage of the Pleistocene period clement winters and cool summers permitted the wide diffusion and intimate association of plants which have now a very different range. Temperate and southern species, like the ash, the poplar, the sycamore, the fig-tree, the Judas-tree, etc., overspread all the low grounds of France as far north at least as Paris. It was under such conditions that elephants, rhinoceroses, hippopotamuses, and vast herds of temperate cervine and bovine species ranged over Europe, from the shores of the Mediterranean up to the latitude of Yorkshire and possibly even farther north still, and from the borders of Asia to the Western Ocean. . . .
"But during another stage of the Pleistocene period the climate of our continent presented the strongest contrast to those general conditions. At that time the dwarf birch of the Scottish Highlands and the Arctic willow, with their northern congeners, grew upon the low grounds of Middle Europe. Arctic animals, such as the musk-sheep and the reindeer, lived then all the year round in the south of France; the mammoth ranged into Spain and Italy; the glutton descended to the shores of the Mediterranean ; the marmot came down to the low grounds at the foot of the Apennines; and the lagomys inhabited the low-lying maritime districts of Corsica and Sardinia. The land and fresh-water moluscs of many Pleistocene deposits tell a similar tale: high alpine, boreal, and hyperborean forms are characteristic of those deposits in Central Europe; even in the Southern regions of our continent the shells testify to a former colder and wetter climate. It was during the climax of these conditions that the caves of Aquitaine were occupied by those artistic men who appear to have delighted in carving and engraving.
"Before considering the evidence supplied by the Pleistocene deposits in the extra-glacial tracts of Central Europe, it may be well to indicate shortly what relation our cave-accumulations and valley-drifts bear to our glacial and fluvio-glacial deposits. Now, in the first place, it is obvious that the valley gravels and cave-accumulations contain essentially the same kind of mammalian remains—the northern and temperate and southern forms all belong to the Pleistocene period. In a word, the valley- and cave-deposits are approximately contemporaneous. Then, in the next place, it is not less obvious that the mammalian fauna of the interglacial beds is practically identical with that of the caves and valley-drifts. The presumption, therefore, is that our cave-accumulations and valley-drifts are the equivalents in time of our glacial and interglacial deposits." (Great Ice Age, 3rd ed., pp. 642-4.)
With regard to these extracts I would just observe that both Boyd Dawkins and Geikie, while agreeing on the intermingling of the northern and southern fauna, give no indications of the stage in the sequence of glacial phenomena in which this intermingling took place. As an interglacial warm period is necessarily bounded at both ends of its duration by cold periods,
