The lowering of the temperature which culminated in the glacial period has left palpable traces behind in the changes which it caused in the European fauna. As the pleiocene climate became colder, the animals unfitted to endure the cold, such as the deer of the Indian types of Axis and Rusa, either migrated to the south or became extinct, while their feeding-grounds were invaded by the dwellers in the temperate zone, the stag, roe, bison, and other animals. These in their turn were pushed forward by the arctic group of animals, the musk-sheep, lemming, reindeer, and others, the progress being in the main steadily to the south while the cold was increasing, and the retreat being steadily to the north while it was decreasing. It will follow from this, that the same district in central or north-western Europe would be traversed by these migratory bodies of animals, both in their southern advance in preglacial and glacial times and their northern retreat in postglacial times, and that, therefore, their fossil remains cannot afford a means of fixing the preglacial, glacial, or postglacial, age of the deposit in which they are found, where it is not marked by traces of glaciation. Sir Charles Lyell’s view, that the glacial period cannot be taken as a landmark in the classification of the European pleistocene deposits, is fully borne out by the facts, and still less can it be taken as a hard and fast line between one fauna and another. It cannot be considered a life-era like the eocene, meiocene, pleiocene, or prehistoric divisions of the tertiary period. _Bone-caves inhabited before and after Ice Period._ If we allow that the lowering of the temperature was the principal cause of the presence of temperate and arctic animals, in a region before inhabited by species fitted to live in a comparatively warm climate, it will follow that bone-caves cannot be said to be either pre- or postglacial, by an appeal to their fossil mammalia. If they were open before the minimum of temperature was reached, they would afford shelter to the animals then in the neighbourhood, and they would continue to be occupied in the south during the vast period of time represented by the enormous physical changes in the region north of the line of the Thames, during the development of the ice-sheet, the submergence and the re-elevation of nearly the whole of Britain and Ireland. As, however, the cold increased, the percentage of arctic animals would also increase, and the more temperate species be weeded out. For these reasons it has seemed to me, that the machairodus of Kent’s Hole, and the _Rhinoceros megarhinus_ of Oreston, represent an early stage of the pleistocene period, before the arctic mammalia were present in full force in the caves. It is very probable that vast herds of reindeer lived in the south of France, while northern Britain lay buried under the ice-sheet, as well as during the two succeeding physical changes.