20 JAMES PERRIN SMITH 
These periodic shiftings of ancient geographic relations do not 
show shiftings of the ancient seas, but rather prove the periodic open- 
ing and closing of the gateways connecting them. These gate- 
ways, or portals, are areas of depression on or between continental 
masses, and lie in regions of permanent instability of the earth’s 
crust, where mountain-building, and the accompanying volcanic 
and earthquake disturbances have been prevalent. Some of these 
ancient portals are temporarily open now, on account of recent 
subsidence, as, for instance, the Bering Strait; another has been 
recently opened, by the activity of man—the Suez Canal. And the 
Panama Canal will restore a connection between the Atlantic and 
Pacific, which is temporarily closed on account of recent elevation 
in that region. One of these portals, that connecting the Oriental 
Tethys with the southern waters, has expanded into the Indian 
Ocean. Others of the ancient portals are now concealed in conti- 
nental masses, and their very existence would not be suspected 
without studies in interregional relationships of faunas. 
