CHAP. VII.] 
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 this 
low tract, which 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 birth-place 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 Lemuridse and Hapalidse, 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 Hapalidte and Cebidae, 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 Cebidae 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 
