GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 235 



very graphically the effect of the great central arid 

 region and lofty mountains in preventing the migration 

 of plants from one side of the continent to the other. 



As we have already intimated, it is evident from the 

 geological record that in Tertiary times the northern 

 regions enjoyed a much milder climate than at present, 

 this being shown by the character of the fossil remains of 

 both animals and plants. Many of the common Tertiary 

 types of plants are now represented by a small number 

 of their descendants restricted to a much smaller range, 

 like the species of Torreya and Sequoia. In Europe 

 we find these forms associated with many others, like 

 the magnolias, tulip-trees, hickories, and many more, 

 still existing in eastern Asia and America, but else- 

 where extinct. In short, the Tertiary flora of the sub- 

 polar zone was made up mainly of types still existing 

 in regions much further south. The modern descend- 

 ants of these Tertiary plants have many of them per- 

 sisted unchanged in some regions, but have been quite 

 crowded out or very much modified in others. The 

 retreat of these plants from their northern habitat was 

 mainly due, no doubt, to great climatic changes, and 

 the principal of these was the severe glaciation to which 

 the whole northern part of the globe was subjected in 

 post-tertiary times. 



As the ice-sheet advanced southward, the plants were 

 driven before it, and many forms were doubtless com- 

 pletely destroyed. The fate of these progenitors of the 

 existing flora of the northern hemisphere was very dif- 

 ferent in different parts of the earth. In America and 

 eastern Asia the trend of the main mountain ranges is 

 north and south, and offered no barrier to the south- 



