THE AMERICAN FAUNA AND ITS ORIGIN. 
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from the Arctic regions through the North Temperate zone 
through the tropics and through the South Temperate zone far 
towards the Antarctic Circle. But in addition to the latitudinal 
extension of America, its far-reaching and strikingly different 
physical features with great range of hygroscopic characters 
over widely separated areas, all contribute to the variety of 
conditions giving great climatic diversity. Mountains extend 
continuously throughout the entire length, rising in some of 
their peaks to over 20,000 feet, and enclosing almost rainless 
areas, while low plains with innumerable rivers and streams give 
regions abounding with water and atmospheric humidity little 
above sea-level. 
The range of climatic and physical conditions that affect 
animal life may therefore be said to be as great within the 
bounds of the American continent as in the whole world, for 
the superior elevation of the Himalayas over the Andes is above 
the altitude at which animal organisms live, since even the 
adventurous mountaineers who have recently ascended the 
Himalayas to over 23,000 feet could not continuously live on 
the summits they so arduously gained. 
Were the American portion of the land area of the globe 
continuous with the other portions, there would be nothing 
remarkable in finding in the western, a diversity of animal life 
equal to that of the Eastern Hemisphere, nor would it be 
surprising to find all the types of the one represented in the 
other. But the interposition of water as separating seas is such 
a formidable barrier to the migration of land animals, that the 
subject of the fauna of America acquires therefrom additional 
interest both with regard to its present features and to the 
indications its consideration affords of past geographical 
conditions and geological changes. 
America cannot, however, be regarded as a single zoo- 
geographical area, since zoologists have long been impressed by 
the great difference between the general character of the fauna 
of its southern and that of the fauna of its northern extension, 
and they have therefore regarded these two portions as distinct 
zoological provinces or regions. In accordance with this view, 
Dr. Sclater constituted them two of his six zoological regions 
of the globe. The southern part of America, but including the 
West Indian Islands, Central America, and the eastern and 
western parts of Mexico, was named the Neotropical region, and 
the whole of the remaining part of the continent, the Nearctic 
region. 
The subject of the distribution of the American fauna and 
