The Classification of the Pleistocene strata of Britain and the Continent by means of the Mammalia

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Abstract
1. The Principle of Classification . The Pleistocene period was one of very long duration, and embraced changes of great magnitude in the geography of Europe. The climate, which in the preceding Pliocene age, in Northern and Middle Europe, had been temperate, at the beginning of the Pleistocene gradually passed into the extreme arctic severity of the glacial period; and this change caused a corresponding change of the forms of animal life, the Pliocene species (whose constitutions were adjusted to temperate or hot climates) yielded place to those which were better adapted to the new conditions; and since, as we shall presently see, there is reason for the belief that it was not continuous in one direction, but that there were pauses, or even reversions towards the old temperate state, it follows that the two groups of animals would at times overlap, and their remains be intermingled with each other. The frontiers also of each of the geographical provinces must necessarily have varied with the season; and the competition for the same feeding-grounds, between the invading and the retreating forms, must have been long, fluctuating, and severe. The passage, therefore, from the Pliocene to the Pleistocene fauna must have been extremely gradual in each area; and the lines of definition between the two must be, to a great extent arbitrary, instead of being sufficiently strongly marked to constitute a barrier between the Tertiary and Posttertiary groups of life of Lyell, or between the Tertiary and Quaternary of the French geologists. The

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