NO. 5 MAMMALS OF PANAMA GOLDMAN 23 



autumnal appearance, relieved to some extent along the borders of 

 streams. It is in this dry forest that one notes the strange habit, 

 possessed by various unrelated species, of producing flowers and 

 ripening fruits while the trees are in a leafless condition. 



FAUNAL RELATIONS 



The geological structure and history of Panama and Central 

 America in general are, as yet, very imperfectly known. The at- 

 tenuation of the isthmian region and the slight elevation of various 

 trans-isthmian passes, irrespective of other data, suggest the prob- 

 able former isolation of the two greater Americas. Some of the 

 passes are less than 500 feet above sea level, and a subsidence of 1,000 

 feet of the present continental mass would establish interocean con- 

 nections at various points. Beginning on the south some of these 

 are marked by gaps in the mountains at the source of the Rio 

 Napipi, a tributary of the Rio Atrato, at the Sucubti, an affluent of the 

 Rio Chucunaque, at the Canal Zone, and farther north at Lake 

 Nicaragua and at the Isthmus of Tehauntepec. Such a division 

 would leave a chain of islands, several o'f the more southern of which 

 would be 3,000 to 4,000 feet high, and it would isolate the high 

 mountains of Costa Rica and Guatemala. 



Geological investigations, especially those pursued in connection 

 with Panama Canal construction, indicate that oceanic waters did in 

 fact extend across, at least at the Canal Zone, during the Oligocene 

 period ; but the date of land emergence has not been very definitely 

 determined. The slight depth of the water to a submarine escarp- 

 ment far out along the coasts of Panama, and the present rapid rate 

 of erosion, indicate that the Isthmus was formerly much broader 

 than at present. The encroachment of the sea is well shown along 

 much of the northern coast line, where cliffs receive the full battering 

 effect of the waves swept in by the northerly trade winds. Southerly 

 winds are less dominant, but the southern coast is constantly sub- 

 jected to the erosive influence of tremendous tides. 



Coiba Island and the large islands of the Pearl Archipelago lie in 

 shallow water upon the continental shelf and may have formed parts 

 of an ancient mainland. The excessive rainfall and tendency of 

 isthmian rocks in general to disintegrate rapidly on exposure to the 

 elements also greatly accelerate the reduction of the general land 

 mass. 



The Miocene mammalian faunas of southern South America and 

 of North America are known to have been widely different, but a 



