chap, vil] MAMMALIA OF THE NEW WORLD. 153 



tropical climate, and would thus afford an ample area for the 

 continued existence and development of the typical South 

 American fauna ; even had glaciers descended in places so low 

 as what is now the level of the sea, which, however, there is no 

 reason to believe they ever did. It is probable too, that tins 

 low tract, winch all round the Gulf of Mexico would be of con- 

 siderable width, offered that passage for intermigration between 

 North and South America, which led to the sudden appearance 

 in the former country in Post-Pliocene times, of the huge Mega- 

 theroids from the latter ; a migration which took place in op- 

 posite directions as we shall presently show. 



The birthplace and migrations of some mammalian families 

 and genera. — We have now to consider a few of those cases 

 in which the evidence already at our command, is sufficiently 

 definite and complete, to enable us to pronounce with some con- 

 fidence as to the last movements of several important groups of 

 mammalia. 



Primates. — The occurrence in North America of numerous 

 forms of Lemuroidea, forming two extinct families, which are 

 believed by American paleontologists to present generalized 

 features of both Lemuridae and Hapalida?, while in Europe only 

 Lemurine forms allied to those of Africa have occurred in 

 deposits of the same age (Eocene), renders it possible that the 

 Primates may have originated in America, and sent one branch 

 to South America to form the Hapalida? and Cebidas, and 

 another to the Old "World, giving rise to the lemurs and true 

 apes. But the fact that apes of a high degree of organization 

 occur in the European Miocene, while in the Eocene, a monkey 

 believed to have relations to the Lemuroids and Cebidse has also 

 been discovered, make it more probable that the ancestral forms 

 of this order originated in the Old World at a still earlier period. 

 The absence of any early tertiary remains from the tropical parts 

 of the two hemispheres, renders it impossible to arrive at any 

 definite conclusions as to the origin of groups which were, no 

 doubt, always best developed in tropical regions. 



Carnivora. — This is a very ancient and wide-spread group, the 

 families and genera of which had an extensive range in very 



Vol. I.— 12 



