100 ORIGIN, ETC., OF THE FAUNA. 
from Ecuador and Colombia, does not appear to reach Central America; but no fewer 
than five genera of the former occur in our area, namely Didelphys, Metachirus, 
Caluromys (=Philander), Marmosa, and Chironectes. Of these Didelphys is the only 
one that is found in North America, where it ranges from about the 40th parallel 
of N. lat. to Texas, thence passing southwards through Mexico, Nicaragua, Yucatan, 
Guatemala, and Panama as far as the Argentine and Chili. The others are Central 
and South American, Marmosa ranging from Mexico (Oaxaca), Costa Rica, and Panama 
to Chili, Wfetachirus trom Mexico (Vera Cruz), Guatemala, and Costa Rica to the 
Argentine, Caluromys from Mexico (Tabasco) to Paraguay, and Chironectes, the 
aberrant water-opossum, from Guatemala to Brazil. 
Extinct Marsupials belonging to the Didelphyide, and closely related to the existing 
genera of that family, have been discovered in Lower Eocene deposits in North 
America and as late as the Lower Miocene in Europe, and in what are believed to be 
Upper Cretaceous beds in South America, Formerly, therefore, the family was very 
widely distributed. Nevertheless, Opossums appear to have survived uninterruptedly in 
South America, at all events since the earliest Tertiary times, and that country is now 
their headquarters. These facts, coupled with the absence of paleontological evidence 
that the group survived in North America after the Oligoceue and in Europe after the 
Miocene, justify the supposition that the existing genera are recent immigrants from 
South into Central and North America. 
SUMMARY AND CONCLUSION. 
ORIGINS OF THE CENTRAL AMERICAN MAMMALIAN FatNa. 
The foregoing account shows that the Mammalian fauna of Central America is a 
mixture of two elements, namely, of forms which are dominant in North and South 
America respectively ; and the paleontological history, during Tertiary times, of the 
orders concerned, supplies—in part, at all events—the explanation of the intermixture. | 
So far as this history has been read, it furnishes strong evidence that the Mammals 
may be referred to two categories. To the first belong the Insectivora, Carnivora, 
Artiodactyla, Perissodactyla, and the Rodentia (with the exception of the Hystrico- 
morphs and possibly some of the Cricetine Myomorphs), which were evolved 
through long ages in the Northern Hemisphere and inferentially passed from North 
into South America by way of Mexico and Panama. ‘The second comprises the 
Edentates, Primates, Marsupials, and the Hystricomorphous Rodents which, from 
whatever country they may originally have come, have undergone a prolonged course 
of evolution in South America and migrated thence into Central or even North 
America. ‘That is to say, whereas North and South America have been independent 
geographical centres for the evolution of Mammals in the Western Hemisphere, the 
part played in the main by Central America has been that of a bridge joining these 
centres together and permitting the intermingling of the independently developed 
