XIII EVOLUTION AND DISTRIBUTION OF ANIMALS 277 



maximum depth of about 12,000 feet, but with large areas 

 between 5,000 and 10,000 feet deep; and although this 

 implies an immense subsidence, it is not very improbable 

 that all the area from this line northward to Greenland 

 and Iceland was dry land during some part of the Miocene 

 period. In support of this view it may be noted that the 

 Alps, the Pyrenees, the Rocky Mountains, and even the 

 Himalayas, were all in early Miocene times many thousand 

 feet lower than they are now. This is proved by the fact 

 of Eocene and Miocene marine deposits of great thickness, 

 which must have been formed in rather deep water, being 

 found elevated from ten to sixteen thousand feet above 

 the sea-level. As an example we may mention the Dent 

 du Midi in Switzerland, where marine shells of early 

 Miocene or late Eocene type are found at an elevation of 

 10,940 feet; and, as this mountain must have suffered 

 enormous denudation, these figures can only represent a 

 portion of the rise of the land, most of which occurred 

 during the Miocene period. To balance this rise over 

 extensive areas on both sides of the Atlantic, there must 

 have been corresponding areas of subsidence, and we may 

 fairly locate these where the indications of palaeontology 

 and geography concur in rendering them probable. We 

 have already seen that the migrations of mammalia 

 between Europe and America have been such as to render 

 some land route necessary,^ while the broken-up character 

 of the coasts of Ireland and Newfoundland, Labrador, 

 Greenland, and Iceland, with the extensive bank of the 

 Azores, all point to a certain amount of recent sinking of 

 land on the outskirts of this area of great depression. 



To Dr. Sclater's question — Where did the tropical land 

 exist which afforded the passage of the tropicopolitan 

 forms from one continent to the other ? — it may therefore 

 be answered: It existed in the north temperate zone 

 during some part of the Miocene period, at the time 

 probably when a rich warm temperate flora covered what 

 are now the icy wastes of Greenland and Spitzbergen. In 

 the North Atlantic a continuous land may have united 



^ For particulars of these migrations see the writer's Geographical 

 Distribution of Animals, vol. i. pp. 140, 153. 



