MIGRATION OF TERRESTRIAL VERTEBRATES 295 



lished easy communications for terrestrial and 

 fresh- water animals. 



Thanks to these early phenomena of migration, 

 of which it is, however, difficult for us at the 

 present day to settle the exact direction, we are able 

 to mark out very clearly, towards the border line of 

 Primary and Secondary times, the existence of two 

 Continental masses spreading from east to west, 

 that is to say, in the converse direction to that of 

 the present great Continents : (1) a Boreal Mass 

 comprising the North of America, Greenland, the 

 Northern regions of the Atlantic, the British Isles, 

 Scandinavia, Russia as far as the Ural, and then, on 

 the other side of an arm of the Sea of Ural, an 

 Asiatic land which was Ed. Suess's Continent of 

 Angara ; and (2) an Austral Mass, extending from 

 Australia to the Indian peninsula and to South 

 Africa, which, doubtless, was prolonged across the 

 Atlantic to South America. This was the Great 

 Continent of Gondwana. 



Between these two great masses there existed 

 a vast arm of the sea, going from Central America 

 to Indo-China across the Atlantic and the Medi- 

 terranean, in an almost equatorial direction ; this 

 was the Great Central Mediterranean of Neumayr, 

 the Thethys of Ed. Suess, the Mesogea of modern 

 palseogeographers . 



This Mesogea was not, however, always an in- 

 superable obstacle to the migrations of terrestrial 

 animals. At certain epochs, which correspond to 

 the most energetic phases of the wrinkling of the 

 earth's crust, bridges have been, so to speak, 

 momentarily thrown across between the Austral 



