18 



PROCEEDINGS OF THE NATIONAL MUSEUM 



endemism can be explained in two ways: chance isolation (with 

 possible subsequent divergence) of a species with a former, wider 

 distribution (relicts) ; and arrival of a species with subsequent estab- 

 lishment of breedino; populations and divergence before other possible 

 competitors have had the chance to invade and become established. 

 The central Pacific of the past probably was much more densely 

 spotted with islands than it is today (see Menard, 1964, especially 

 his discussion of the Darwin Rise). Various factors (subsidence, 



