288 ANNUAL REPORT SRHTHSONIAN INSTITUlTION, 1951 



the late Pliocene conditions were essentially as they are today, but 

 Newfoundland was still united with North America and most of 

 Florida was submerged. In the Pleistocene, Alaska was again broadly 

 connected Avith Asia, and Newfoundland became separated from the 

 mainland. 



In spite of relatively minor changes in the relation of sea and land, 

 North America has always been essentially an integral part of tlie 

 great permanent land mass of the Northern Hemisphere, the nursery 

 of the world's land fauna from which at various times in the past 

 the different animal types spread southward. There is no geological 

 evidence of any fundamental changes in the relation between the 

 oceans and the land masses. 



A glance at a physical map of the Americas shows that from Guate- 

 mala to northern Venezuela, including the West Indies, the general 

 trend of the highlands is east and west in marked contrast to the moun- 

 tains of North and South America, which run approximately north 

 and south. Judged solely on the basis of the Recent fauna, Central 

 America may be defined as the region from the southern end of the 

 Mexican Plateau to northern Venezuela, including the "West Indies. 

 But there is no geological or paleontological evidence that Central 

 America was ever a geographical entity apart from tropical America 

 as a whole. The concept of a hypothetical Antilia including the 

 Central or Middle American area which, in one form or another, has 

 been widely used to explain the differentiation and distribution of 

 animal life in this region has no foundation in fact. Islands now 

 separate were joined with each other, or with the mainland, but there 

 was never a large and continuous land mass in the region between 

 North and South America. 



From the uniqueness of its fauna, it is evident that South America 

 was for a long time wholly or in part isolated from the rest of the 

 world, and this isolation is emphasized especially by the abundance 

 in the past of large and bizarre mammals now known only as fossils. 

 It is generally agreed that the eastern highlands of the Guianas and 

 Brazil have been above the sea since the Trias, and that at some time 

 or other in the Tertiary there was an oceanic interconnection along the 

 Amazon Valley, or a long gulf extending inward from the Atlantic 

 or Pacific, probably the latter. It is believed that Chile and the Pata- 

 gonian Andes, which are quite distinct from the more northern Andes, 

 have been land since very early times, and most students agree that 

 southern South America was joined through Antarctica with New 

 Zealand and Australia, at least until the Cretaceous. In the early 

 Eocene, South America was cut off from North America. Between the 

 late Eocene and the Pliocene there were various islands between the 

 two. In the late Pliocene they were again joined, and a considerable 

 interchange of faunas followed. 



