MAMMALIA. 101 
faunas. A few genera of Chiroptera and Rodentia restricted, so far as is known, to 
Central America, may have been evolved within the area; but these do not alter the 
general character of the country as a faunistically transitional tract between the 
northern and southern portions of the Western Hemisphere. | 
That the intermingling and cross-migrations, which give the stamp to the existing 
Central American Mammalian fauna, began in the Miocene and have continued 
uninterruptedly since that date may be inferred from paleontological and geological 
data, which support the conclusion that the Northern and Southern Americas, separated 
by sea over what is now the isthmus of Panama from the earliest Eocene (Paleocene) 
to the end of the Oligocene, were finally joined by the elevation of that isthmus during 
the Miocene Period. 
Possibly, as held by some authorities, Central America to the north of Costa Rica 
and Panama was temporarily connected with South America by way of the Greater 
and Lesser Antilles in the Early Oligocene; and possibly there was a stiil earlier union 
during the Cretaceous of the western portions of North, Central, and South America. 
However that may be, it seems tolerably certain that the main streams of migration 
passed by way of Costa Rica and Panama in comparatively recent Tertiary times. 
Reference has more than once been made to the possibility of South America 
having received the ancestors of some of its characteristic forms of Mammals (Platy- 
rhine Monkeys, Hystricomorph Rodents, Sirenians) from Africa by a direct trans- 
Atlantic bridge between the two continents. Other groups of animals supply evidence 
for the existence of this union. But if the mammals made use of it, it must have 
endured into early Tertiary times. This, however, has been disputed. 
Another point, also under discussion, may here be referred to, although it has no 
direct bearing, so far as is known, upon the fauna of Central America. This is the 
possibility of a direct connection between South America and Australia. Perhaps the 
most cogent evidence for this is supplied by certain genera of invertebrates, probably 
in great part ancient types. So far as Mammals are concerned, the evidence rests 
upon the claimed relationship between the so-called Tasmanian or Marsupial Wolf 
(Lhylacinus) and some extinct (Upper Miocene) Sparassodont Mammals of South 
America, coupled with the later date of fossil Marsupials in Australia, the absence of 
their remains in Tertiary deposits in China, and the absence of living forms in South- 
east Asia to the west of “ Wallace’s Line.” Also there is the undeniable kinship 
between the Australian Dasyurids and the American Didelphyide, of which Marmosa 
is alleged to be the most primitive type. It would be out of place further to discuss 
these questions here; but the facts, as they stand, are suggestive of the origin of the 
Australian Marsupials from a South American stock. Moreover, if Thylacinus be of 
the same family as the Sparassodont Prothylacinus, it may be held as perhaps 
probable that the connecting land-mass permitting the migration persisted into early 
‘Tertiary times. 
