ENVIRONMENT AND EVOLUTION OF VERTEBRATES ^0 



jection the intimate relation to each other of the land sur- 

 faces is not apparent. Indeed from a zoological standpoint this 

 map is useless. On the other hand if one look at a map on 

 north polar projection (Fig. 1) one sees immediately that Asia 

 and America are closely related across the Bering Strait and 

 also by way of Europe and Greenland. Nowhere is the sea 

 deeper than some 600 feet. If, therefore, the land were raised 

 by this amount there would be actual continuity between these 

 various land masses and animals would be free to roam from 

 one to the other provided the conditions of climate were not 

 unfavorable. Very different is the case in the southern hemi- 

 sphere. Around Antarctica is a sea of great depth separating 

 this land from Australia, South Africa and South America and 

 also isolating these various land masses from each other. No 

 gentle undulation, no slight rise of the earth's surface could 

 obliterate this sea. It is doubtful if these land masses have ever 

 been connected; if animals could pass from one to the other at 

 any time except across the ocean. 



The alternate rise and fall of the land with consequent with- 

 drawal and encroachment by the sea is known by the term 

 Isostatic Balance.* Around each continent, extending under 

 the sea for a variable distance, is the continental shelf. The 

 mountains and the higher lands are constantly Avashed away 

 by river action or by glaciers and the sediments deposited in 

 the shallow water upon the continental shelf. Thus land is 

 formed at the mouths of rivers, being transported from its 

 original situation in the interior. The weight of this made land 

 is supposed to have some influence in causing the crust of the 

 earth beneath it to sink and in consequence to bring about the 

 elevation of the land remote from the coast. Whether or no 

 this theory be justifiable we know at least that land is con- 

 stantly removed and as constantly raised anew. With these 



*It is beyond the province of this volume to consider the arguments in support and 

 in controversion of the vexed theory of Isostatic Balance. Those who are specially 

 interested in the relation of this theory to the problems of mammalian evolution and 

 migration cannot do better than study carefully the papers by Matthew: Climate 

 and Evolution, Ann. New York Acad. Sc, 1915, xxiv, 171; and Barbour: Some 

 Remarks upon Matthew's "Climate and Evolution," Ann. New York Acad. Sc, 1916, 

 xxVii, 1. 



