MOUNTAIN ANTS OF NORTH AMERICA. 497 



the limits of which occurs a more generalized type of the same organ- 

 ism. The subsequent evolution of additional types, which will most 

 frequently occur at or near the so-called center of distribution as a 

 natural result of the greater facility for adaptation due to the greater 

 distance apart of the physico-economic barriers and tlie consequently 

 greater radius of each type, will result in the gradual formation of a 

 dispersal figure which would be ideally represented by a series of con- 

 centric circles, each of the circles representing a barrier, the small 

 central circle enclosing the most perfected type and the peripheral 

 band the most generalized, the intervening areas including intermedi- 

 ate types increasing in specialization toward the center." 



The Formicidae show in a very striking manner the relegation of the 

 most primitive forms to the tropics and southern hemisphere and 

 especially to the Neotropical, Oriental and Australian regions. As all 

 of these forms are exquisitely thermophilous and stenothermal, 

 whereas the Palearctic and Nearctic faunas and particularly the forms 

 peculiar to the mountains consist of more specialized, stenothermal 

 and psychrophilous species together with a small number of eury- 

 thermal ubiquists, we are led to believe that the development of zonal 

 climates during the Tertiary has been the essential factor in determin- 

 ing the distribution of the present world-wide distribution of ants. 

 The mountain faunas are therefore of comparatively recent origin and 

 this is particularly true of that of the Rocky Mountains, to judge from 

 the large number of subspecies and varieties, most of which have, in 

 all probability, developed since the Pleistocene. The Rocky Moun- 

 tains as an independent center of formation of new forms contrast 

 markedly with the Alps and Himalayas, for there are relatively few 

 ants peculiar to the latter and especially to the Alps. This may be 

 attributed to geological conditions. Geologists maintain that the 

 Rocky Mountains began to be elevated as early as late Cretaceous 

 time and by the Eocene had attained altitudes of 4000 to 5000 feet. 

 They continued to rise during the Tertiary Period to altitudes of 

 13,000 to 14,000 feet, with a corresponding elevation of the bases 

 between them and considerable erosion of their summits. The Alps 

 however, did not appear till the close of the Oligocene and only during 

 the Miocene were the Himalayas uplifted. The Alpine area, more- 

 over, was surrounded by water till the Miocene when it became joined 

 by a broad land-connection with Central Asia. Its connection with 

 France by means of another land-connection is said to have occurred 

 at the end of the Miocene. These conditions, together with the later 

 extensive glaciation of the Alps, must have been very potent factors 



