Classification of Pleistocene Strata by means of the Mammalia.--The late, middle, and early Pleistocene Divisions.--The Pleiocene Mammalia.--Summary of characteristic Pleiocene and Pleistocene Species.--Antiquity of Man in Europe.--Man lived in India in Pleistocene Age.--Are the Palæolithic Aborigines of India related to those of Europe?--Palæolithic Man lived in Palestine.-- Conclusion. The animals inhabiting the caves have been enumerated in the last three chapters, and we have discussed the inferences drawn from their distribution as to the pleistocene climate and geography of Europe. It remains for us now, in conclusion, to define the pleistocene, and to see in what relation it stands to the pleiocene period. _Classification of Pleistocene Strata by means of the Mammalia._ The pleistocene period was one of very long duration, and embraced changes of great magnitude in the geography of Europe, as we have seen in the ninth and tenth chapters. The climate, which in the preceding pleiocene age had been temperate in northern and middle Europe, at the beginning of the pleistocene gradually passed into the extreme arctic severity of the glacial period. This change caused a corresponding change of the forms of animal life; the pleiocene species, whose constitutions were adjusted to temperate or hot climates, yielding place to those which were better adapted to the new conditions. And since 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 naturally have varied with the season; and the competition for the same feeding-grounds between the invading and retreating forms must have been long, fluctuating, and severe. The passage, therefore, from the pleiocene to the pleistocene fauna might be expected to have been extremely gradual in each area. The lines of definition between the two are to a great extent arbitrary, instead of being marked with sufficient strength to constitute a barrier between the tertiary and post-tertiary groups of life of Lyell, or between the tertiary and quaternary of French geologists. The principle of classification which I have proposed[267] is that offered by the gradual lowering of the temperature, which has left its mark in the advent of animals before unknown in Europe; and according to it I have divided the pleistocene deposits into three groups. 1. Those in which the pleistocene immigrants had begun to disturb the pleiocene mammalia, but had not yet supplanted the more southern animals. No arctic mammalia had as yet arrived. To this group belongs the forest-bed of Norfolk and Suffolk, and the deposit at St. Prest, near Chartres.