MOUNTAIN ANTS OF NORTH AMERICA. 491 



Paleographers agree in characterizing the Upper Jurassic as a 

 period of great continental emergence, warm climate and a cosmopoli- 

 tan flora. During this and the ensuing Cretaceous most of the families 

 and genera of insects, like the flora on which so many of them were 

 vitally dependent, must have assumed their modern facies and have 

 become very widely distributed. According to Osborn (1910, p. 95) 

 the "most memorable fact about the flora is one recently insisted 

 upon by Knowlton (1909), namely that as we pass from the Cretaceous 

 into the Eocene there is no appreciable change in the flora. From 

 this it would appear that there was no secular change of climate; 

 that the temperature was the same." There is nothing to indicate 

 that the insects underwent any profounder change than the plants, 

 so that we are unable to believe that these animals exhibited anything 

 like the catastrophic elimination which occurred in several other groups 

 of organisms both terrestrial and aquatic at the end of the Cretaceous. 

 There is therefore no justification for assuming a close parallel in the 

 course of development of such insects as the ants during the Tertiary 

 with that of the mammals, whose phylogeny during that period was very 

 complicated and greatly accelerated. The repeated migrations of 

 mammals between North America and Eurasia during Cretaceous and 

 Posteocene time were probably paralleled by the ants but we have 

 no precise evidence of such movements. A single land-bridge, the 

 Siberian-Alaskan, which is accepted by all students of geographical 

 distribution, and according to most of them was in existence during 

 the Cretaceous and again from late Miocene to Pleistocene times, 

 is sufficient to account for the present constitution of our North 

 American ant-fauna. Scharff (1907, 1912) and others have adduced 

 considerable evidence in favor of another land-bridge connecting 

 North America with Great Britain and Scandinavia during Pre- 

 glacial and early Glacial time, but others reject this construction 

 though they have not succeeded in accounting for the fauna and flora 

 of Greenland and Iceland and the distribution of many eastern Nearc- 

 tic and western European forms on any other hypothesis. That 

 there was a gradual cooling of the climate from late Eocene to the 

 Glacial Epoch is also generally admitted and the resulting develop- 

 ment of pronounced zonal climates had a very powerful effect, as we 

 know, on the fauna and flora of the northern hemisphere. The elimi- 

 nation of species thus induced over the area covered by the great 

 ice-sheet both in Europe and North America and the southward 

 migration of surviving species away from its border have been so 

 often discussed that I need not dwell on them here. The ants of the 



