THE GEOGRAPHICAL DISTRIBUTION OF AXTS. H9 



second was represented by distinct eastern and western coniferous 

 forest types, and the third by the biota of the southeastern and south- 

 western states. The first wave was of a trans-continental extent, the 

 second while coniferous and transcontinental was composed of two 

 distinct types, the eastern, represented by the biota of northeastern 

 North America, and the western by that of the Rocky Mountains and 

 the Pacific Coast. The northeastern biota overflowed to the north, 

 to the northwest into the Mackensie basin and even a few forms into 

 the Yukon valley and to the Rocky Mountains. The northwestern 

 biota spread from the Rocky Mountains and Pacific Coast region in 

 the United States north to British Columbia and Alaska. The third 

 wave spread from the southeastern centre of dispersal northward to 

 the conifers and west to the Great Plains. From the southwestern 

 centre the life spread north on each side of the Rocky Mountains into 

 Canada, and only stragglers spread eastward into the humid region." 



Besides the three waves recognized by Adams it is necessary to 

 recognize a fourth or tropical wave of species, which have been moving 

 up into North America from South America. As this has come over 

 two distinct routes, namely by way of the West Indies and Mexico, 

 we may recognize an eastern and a western center as in the second 

 and third waves. 



At present our knowledge of the ants of British America and 

 Alaska is so incomplete that it is impossible to state whether there is 

 a distinct tundral fauna, that is, a fauna living beyond the coniferous 

 tree-belt. Observations in the mountains of Colorado, however, indi- 

 cate that ants do not occur far above timber-line, which is there at 

 an altitude of about 4,000 meters. Isolated females may sometimes be 

 found under stones at a greater elevation, but these have been borne 

 aloft by air-currents and perish without being able to establish formi- 

 caries. This is also the case at more moderate elevations above the 

 timber-line, as, for example, on the summit of Mt. Washington 

 (Wheeler, 1905/0. 



Even the non-glaciated portion of North America, however, has 

 retained an ant-fauna composed very largely of well-known Eurasian 

 genera and relicts of a more southerly type which have been unequally 

 preserved in the eastern and western portions of the United States. 

 The eastern portion retained a very small number of these ancient 

 genera, probably on account of its much colder climate during the 

 glacial epoch. Nevertheless, the eastern and western centers of the 

 areas covered by Adams's second and third waves each retained a cer- 

 tain number of relicts, which seem to have formed as many constella- 

 tions of species, subspecies and varieties within comparatively recent 



