THE TERTIARY TO THE MODERN PERIOD. 223 



refuge to plants in times of submergence, and means of 

 escape to the south in times of refrigeration. Hence, 

 the greater continuity of American vegetation and the 

 survival of genera like Sequoia and Liriodendron, which 

 have perished in the Old World. Still, there are some ex- 

 ceptions to this, for the gingko-tree is a case of survival in 

 Asia of a type once plentiful in America, but now extinct 

 there. Eastern Asia has had, however, some considerable 

 share of the same advantage possessed by America, with 

 the addition, referred to by Gray, of a better and more 

 insular climate. 



But our survey of these physical conditions can not be 

 considered complete till we shall have considered the 

 great Glacial age of the Pleistocene. It is certain that 

 throughout the later Miocene and Pliocene the area of land 

 in the northern hemisphere was increasing, and the large 

 and varied continents were tenanted by the noblest vege- 

 tation and the grandest forms of mammalian life that the 

 earth has ever witnessed. As the Pliocene drew to a 

 close, a gradual diminution of warmth came on, and 

 more especially a less equable climate, and this was ac- 

 companied with a subsidence of the land in the temperate 

 regions and with changes of the warm ocean-currents. 

 Thus gradually the summers became cooler and the 

 winters longer and more severe, the hill-tops became 

 covered with permanent snows, glaciers ploughed their 

 way downward into the plains, and masses and fields of 

 floating ice cooled the seas. In these circumstances the 

 richer and more delicate forms of vegetation must have 

 been chilled to death or obliged to remove farther south, 

 and in many extensive regions, hemmed in by the advance 

 of the sea on the one hand and land -ice on the other, they 

 must have altogether perished. 



Yet even in this time vegetation was not altogether 

 extinct. Along the Gulf of Mexico in America, and in 

 the Mediterranean basin in Europe, there were still some 



