We come next to the very interesting question of alternations of climate during the Ice Age. The evidence under this head has accumulated to such an extent within recent years as to convince most students of Pleistocene geology that very extensive changes of climate characterised the glacial period. How many such changes took place we are not yet in a position to say, but we know that the intensely arctic condition of things which has just been described was interrupted more than once by what have been termed "interglacial epochs," during which a mild and genial climate prevailed over middle and northern Europe. For some time it was believed that such "interglacial epochs" had only a local significance, that they bespoke mere transitory retreats of the ice-fields, such as are known to have taken place within historical times in the glacier-valleys of the Alps. But increased observation and reflection have shown that this explanation of the phenomena of "interglacial beds" will not suffice. It is impossible to enter here upon details, but I may briefly state that the evidence in question is two-fold. _First_, we have the stratigraphical evidence. We have ascertained the existence, over wide areas in this and other glaciated countries, of several successive sheets of boulder-clay, which are often separated from each other by fossiliferous aqueous strata. It has been demonstrated that each of these sheets of sub-glacial detritus is the accumulation of a separate and distinct ice-flow. _Second_, we have the evidence of fossil organic remains. We find, for example, that the flora which covered the low-grounds of middle and temperate Europe during a certain stage of the glacial or Pleistocene period, consisted of plants which are now restricted to the tops of our mountains and to northern Scandinavia. The characteristic fauna associated with that flora embraced the reindeer, glutton, mammoth, woolly rhinoceros, Arctic fox lemming, chamois, and so forth. We know, indeed, that man hunted the reindeer and the mammoth in the south of France. Similar testimony to the coldness and humidity of the climate is borne by the land- and freshwater shells which occur in certain Pleistocene deposits in Italy, Corsica, southern France, Switzerland, Germany, etc. That this flora and fauna were contemporaneous with the great glaciation of our Continent has been as well ascertained as the fact of the Roman occupation of Britain. But if the evidence of organic remains strongly confirms and supports that supplied by the distribution of glacial deposits in Europe, no less forcibly does it corroborate the physical evidence as to the former existence of a warm and genial interglacial climate. During interglacial times a most abundant mammalian fauna roamed over all temperate Europe--a fauna comprising such animals as Irish deer, urus, bison, horse, stag, saiga, brown bear, grisly bear, several species of elephant, rhinoceros, and hippopotamus, hyæna, lion, leopard, etc.