which took place in the Pleistocene age,[1] when the Sahara Desert was covered by the sea, and a large part of the Mediterranean area was dry laud, or like that now taking place in the Scandinavian peninsula, which is being lifted up from the North Cape to Stockholm, and depressed in the south, in Scania.[2] New conditions of life were produced by these changes, so unfavourable for the Secondary animals, that all the higher forms perished that were unable to compete in the struggle for life with the new invaders, whose presence marks the Eocene period. The invasion of Europe by the placental mammals is the great event which is the natural starting-point for our enquiry into the ancient history of man, since the conditions by which he was surrounded on his arrival in Europe, form part of a continuous sequence of changes, from that remote period down to the present day. Each of these changes in life and geography will be treated in outline in its due place in this work.
The Eocene Classification.
The Eocene period has been defined in the last chapter as that in which the existing orders and families of the placental mammalia appear for the first time. The Eocene marsupials, however, are, as may be expected from their class having appeared in the Secondary age, in a far more advanced stage of evolution, being represented by a living genus,[3] the opossum, as well as by
