I. For Zoographers Only 



Wallace stated, many years ago, that there are two dif- 

 ferent types of islands. Those which he calls oceanic islands 

 have never by any likelihood been connected with other 

 land. A good example is St. Helena. There are others where 

 changes in the earth's crust have broken up large land 

 masses into what are now islands. There has always been 

 a lot of discussion among naturalists as to details, particu- 

 larly in the East and West Indies. 



It is clear that some separations can be explained by the 

 fact that the oceans stand at a higher level now than they 

 did when a large part of the water on the earth's surface 

 was tied up in the form of ice during the several periods of 

 maximum glaciation — when the polar icecap was enor- 

 mously thick. I have argued principally concerning the 

 West Indies, where many connections could be explained 

 by this tie-up-of-ice theory, and I also believe that many 

 of the deep passageways can be explained by what geol- 

 ogists call downthrust-faulting, where an area drops rap- 

 idly, geologically speaking of course, and makes a deep 

 strait, sometimes counterbalanced by an upthrust some- 

 where else. I believe, for instance, that the mountain 

 known as the Morro of Monte Criste on the northern coast 

 of Hispaniola and the Yunque of Baracoa on the northeast 

 coast of Cuba represent upthrust-fault blocks, while the 

 separation between Jamaica and Haiti represents a com- 

 paratively recent downthrust area. The surface of the 

 earth is in somewhat unstable equilibrium, what geologists 

 call isostatic balance. 



