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- 
