CHAP. VII. ] MAMMALIA OF THE NEW WORLD. 143 
always more or less united to the tropical areas; while the 
evidence of the extinct mammalia themselves shows, that South 
America has been for the most part isolated from the northern 
continent, and did not take part in the development of its 
characteristic Tertiary fauna. 
Before speculating further on this subject, it will be well to 
lay before our readers a summary of South American paleon- 
tology, after which we shall be in a better position to draw 
correct inferences from the whole body of the evidence. 
South AMERICA. 
Unfortunately, our knowledge of the interesting fossil fauna 
of this continent, is almost wholly confined to the Post-Pliocene 
and Pliocene periods. A few remains have been discovered in 
deposits believed to be of Eocene age, but nothing whatever 
representing the vast intervening period, se rich in peculiar 
forms of animal life both in North America and Europe. 
Fauna of the Brazilian caves—What we know of the Post- 
Pliocene period is chiefly due to the long-continued researches 
of Dr. Lund in the caves of Central Brazil, mostly situated in 
a district near the head waters of the San Francisco river 
in the Province of Minas Geraes. The caves are formed in 
limestone rocks, and are so numerous that Dr. Lund visited 
thousands, but only sixty contained bones in any quantity. 
These caves have a floor of reddish earth, often crowded with 
bones. In one experiment, half a cubic foot of this earth 
contained jaws of 400 opossums, 2,000 mice, besides remains 
of bats, porcupines and small birds. In another trial, the 
whole oi the earth in a cavern was carried out for examin- 
ation, amounting to 6,552 firkins; and, from a calculation 
made by measured sainples, it was estimated to contain 
nearly seven millions of jaw-bones of cavies, opossums, por- 
cupines, and mice, besides small birds, lizards, and frogs. This 
immense accumulation is believed to have been formed from 
the bodies of animals brought into the cavern by owls; and, 
as these are unsocial birds, the quantity found implies an 
