144 T. C. CHAMBERLIN 



There are well-known ridges on the bottoms of the oceans 

 and others of minor order will not unlikely yet be disclosed. It is 

 quite consistent with sound dynamics and a conservative attitude 

 to suppose that these may have once been more bowed, or less 

 bowed, than now, and that they may once have cut the sea surface 

 and constituted linear islands, or land bridges, and have played 

 their part in the migrations of plants and animals, just as present 

 bridges may have been once submerged. And these conservative 

 deformations, together with the oscillations and displacements of 

 the segment borders, seem to be about the limit of probable inter- 

 change between the real continents and the real ocean basins. 

 Their dominant feature was, as Dana, Wallace, and others long 

 ago urged, permanency. 



