1915.] Allen, Review of the South American Sciuride. 297 
GEOGRAPHIC DISTRIBUTION AND INTERRELATIONSHIPS OF 
SoutH AMERICAN SQUIRRELS. 
As shown in the foregoing pages, the squirrel fauna of South America, 
embracing some 75 species and subspecies, is separable into 9 natural and 
well circumscribed superspecific groups, here allotted the value of genera; 
some of them include two or more well marked sections. The geographic 
distribution of these groups is correlated with areas markedly different in 
physiographic features. While such correlation is to be expected, it seems 
worth while to point out in some detail the nature and extent of these agree- 
ments. 
The striking topographic features of South America are the highlands of 
the coast borders and the vast extent of the interior lowlands, creating condi- 
tions of environment that result in strong biologic reactions, as most of the 
land area is intertropical. The geologically oldest parts of the continent 
are well known to be the Guianian and Brazilian highlands of the Atlantic 
border, the Andean ranges of the Pacific border having come into existence 
much later, and the interior of the continent remaining an inland sea 
till long after the Caribbean, Pacific, and Atlantic borders had become 
dry land. The present topographic conditions, however, have long pre- 
vailed, and it is these only that we have here especially to consider. 
So far as climatic conditions are concerned, the whole continent of South 
America should be the home of some species of squirrel, the southern extrem- 
ity of the continent being well within the climatic range of squirrels in other 
lands. Butin South America they are of by no means universal distribution, 
as forests are necessary for the existence of tree squirrels, the only members 
of the family Sciuride which have found their way to any part of tropical 
or even subtropical America. Vast areas of South America are forestless. 
More than half of Venezuela consists of treeless plains or Ilanos; large por- 
tions of Brazil are open grassy or scrubby campos, while much the greater 
part of the southern third of the continent consists of pampas, nearly treeless 
steppes, or open chaco country. These immense open spaces are nearly 
as effective barriers to tree squirrels as would be great inland seas. They 
afford them not only no congenial living conditions but interpose barriers 
to their dispersion. 
In general terms, squirrels are restricted in South America to a narrow 
belt near the Caribbean coast, from the Island of Trinidad (geographically 
and faunally a detached fragment of the continent) and the Paria Peninsula 
west to where the Sierra de Mar merges into the Sierra de Merida, and thence 
over the great Andean system of the western border of the continent. 
