WHEELER: ANTS OF THE GENUS FORMICA. 385 
continents, notably in the Rockies and Alleghanies, in the Alps, Cau- 
casus, and Ural Mountains. A similar though less pronounced abun- 
dance of species and colonies is noticeable in the hilly or rolling portions 
of the transition zones of both continents, owing to the similar, though 
somewhat less favorable conditions of temperature, moisture, and vege- 
tation. In more level and arid regions, such as the deserts, the genus 
Formica is replaced by Myrmecocystus in the New, and Cataglyphis 
in the Old World. 
If we divide the total number of known Formicae (144) into Old and 
New World forms, we find that Eurasia possesses only fifty-two, where- 
as North America, though a much smaller land area, possesses ninety- 
three species, subspecies, and varieties.1_ This would seem to indicate 
that the latter continent must be the original home of the genus, 
especially as it possesses representatives of all the Eurasian groups 
of species besides two peculiar to itself (the mzcrogyna group and the 
subgenus Neoformica). Unless we accept the view that the genus 
arose in the polar region during Mesozoic times and radiated its 
species out into Europe, Asia, and North America, we must suppose 
that Eurasia has received its species by immigration from the Nearc- 
tic region. That the latter view 1s the more probable is shown by a 
glance at the distribution of the forms in America. At least thirty- 
nine of our ninety-three forms, or nearly 42%, occur in Colorado and 
the adjacent portions of New Mexico. Not only are these two states 
thus abundantly supplied with species, subspecies, and varieties but 
the colonies of the individual forms are unusually numerous and 
flourishing on the mountain slopes of this territory. We may there- 
fore regard the southern ranges of the Rocky Mountains in the United 
States as the center of origin of the genus and of the dispersal of 
species to other portions of North America. 
Formica thus affords striking confirmation of the views of Adams ? 
and Scharff * that the southwestern states and the adjacent portions 
of Mexico are the seat of one of the most active North American 
centers of species formation and dispersal of both plants and animals. 
It is true that the Formica center does not accurately correspond with 
the southwestern center as defined by Adams for the biota in general, 
since the former lies somewhat further north and is much less arid, 
1One of the species, F. fusca, is counted twice, because it occurs in both hemi- 
spheres. 
2'The Postglacial dispersal of the North American biota. Biol. bull., 1905, 9. 
p. 53-71, 1 fig. 
3 Distribution and origin of life in America. Maemillan Co. 1912. 
