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.^ 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 microgyna 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 xVmerica, we must suppose 

 that Eurasia has received its species by immigration from the Nearc- 

 tic region. That the latter view is 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, 



1 One of the species, F. fusca, is coiinted 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. Macmillan Co. 1912. 



