The climate of Europe had for ages been of so genial a character that sub-tropical types of life had long flourished both in the sea and on the land of this quarter of the globe. But in the period of geological history with which we are now concerned, a remarkable diminution of temperature was in progress all over the Northern hemisphere. As the warmth grew less, the distribution of plants and animals came to be seriously affected. Many southern forms were extirpated from districts which they had long inhabited, while in their place came migrations of northern species. This modification made itself felt both on terrestrial and marine life. Thus in the Atlantic Ocean a number of northern shells, which had pushed their way southward even as far as the coasts of the Spanish peninsula, were able to enter the Mediterranean when a connection was opened between that inland sea and the main ocean outside. It is interesting to note that among the shells introduced into the Mediterranean basin at this time were _Astarte borealis_, _Buccinum groenlandicum_, _Cyprina islandica_, _Panopaea norvegica_ and others whose appellations sufficiently indicate the latitudes where they now find their chief home. On the land, too, such quadrupeds as the reindeer and the now extinct mammoth wandered from the plains of Lapland and Russia to the shores of Italy. Eventually when the refrigeration gave way to the return of more genial conditions, the northern invaders died out. They have no living descendants now in the south of Europe.