TEE EVIDENCE OF SNAILS. 289 



pasturage is to be found. The wide range of such groups as this, and 

 the birds, is limited only by markedly unfavorable physical conditions, 

 and their presence in both the Old and New Worlds merely indicates 

 that up to quite recent times there has been a land bridge over Bering 

 Strait. 



The identical species of plants in Japan and the United States, 

 elaborately discussed by Dr. Asa Gray, are also, in many cases, it would 

 seem, comparatively recent emigrants into one continent or the other, 

 not old enough to have become changed by new surroundings; or 

 they are plants which lived in the Tertiary forests of Greenland and 

 British America, which Heer and others have made known. Through 

 stress of climate, this circumboreal flora has been driven southward, 

 many of its species to be changed by the vicissitudes of the march, 

 while others still flourish unchanged in the two continents. 



