236 EVOLUTION OF PLANTS 



ward retreat of vegetation before the advancing ice- 

 sheet, and as the latter retired again the plants were 

 enabled to return northward. 



In Europe, owing to the position of the great moun- 

 tain chains, as well as its higher latitude, the whole 

 region north of the Alps was subjected to the action of 

 the glaciers, and the southward retreat of the plants being 

 cut off, very many forms perished, while the same plants 

 have survived in the more favored regions of Asia and 

 America, in both of which a far larger number of sur- 

 vivors of the primordial Tertiary flora occur than in 

 Europe. The occurrence of nearly related isolated types 

 in widely separated regions can almost always be ex- 

 plained as a survival from once widely distributed an- 

 cestors. In the case of herbaceous plants, such as 

 Podophyllum. Stylophorum, and other peculiar types 

 common to eastern Asia and Atlantic North America, 

 we can only reason from analogy, but in the case of many 

 woody plants, especially trees, e.g. the tulip-tree (Lirio- 

 dendron), Torreya, etc., this is abundantly proved by the 

 fossil remains. 



Perhaps the most striking instance known of close 

 correspondence in the flora of widely separated regions, 

 is the one already spoken of, i.e. the great number of 

 identical or closely related plants found in the temper- 

 ate regions of Pacific Asia, northern China, Mantchuria, 

 and Japan and Atlantic North America. Much of our 

 knowledge of these extraordinary similarities we owe to 

 the labors of Asa Grav. 



/ 



The writer recalls vividly the strangely familiar aspect 

 of the vegetation of Japan, especially in the northern 

 part, where nearly all of the more noticeable plants 



