At this point, before we deal with the arrival in Britain of the representatives of the early races, it should be noted that differences of opinion exist among scientists regarding the geological horizons of the Palæolithic culture stages. In the Pleistocene Age there appear to have been four great glacial epochs and two minor ones. Geological opinion is, however, divided in this connection.
[Illustration: WESTERN EUROPE DURING THE THIRD INTER-GLACIAL EPOCH
(According to the Abbé Breuil the Strait of Gibraltar was open and the Balearic group a great island.)]
During the First Glacial epoch the musk-ox, now found in the Arctic regions, migrated as far south as Sussex. The Pliocene[9] mammals were not, however, completely exterminated; many of them survived until the First Interglacial epoch, which lasted for about 75,000 years--that is three times longer than the First Glacial epoch. The Second Glacial epoch is believed to have extended over 25,000 years. It brought to the southern shores of the Baltic Sea the reindeer and the hairy mammoth. Then came the prolonged Second Interglacial stage which prevailed for about 200,000 years. The climate of Europe underwent a change until it grew warmer than it is at the present day, and trees, not now found farther north than the Canary Islands, flourished in the forests of southern France. The Third Glacial stage gradually came on, grew in intensity, and then declined during a period estimated at about 25,000 years. It was followed by the Third Interglacial epoch which may have extended over at least 100,000 years. African animals returned to Europe and mingled with those that wandered from Asia and the survivors in Europe of the Second Interglacial fauna. The Fourth Glacial epoch, which is believed to have lasted for about 25,000 years, was very severe. All the African or Asiatic mammals either migrated or became extinct with the exception of lions and hyænas, and the reindeer found the western plains of Europe as congenial as it does the northern plains at the present time.
[9] The last division of the Tertiary period.
During the Fourth Post-glacial epoch there were for a period of about 25,000 years[10] partial glaciations and milder intervals, until during the Neolithic Age of the archæologists the climate of Europe reached the phase that at present prevails.