386 bulletin: museum of comparative zoology. 



but this is, perhaps, a matter of minor importance. Both Adams and 

 Scharff recognize another center of species formation and dispersal 

 in the southeastern states, but none of our Formicae seems to have 

 arisen in this region, although this does not apply to other ant-genera. 

 F. pallidefulva is the only species of the genus that might be supposed 

 to have originated in such a center, but the occurrence of some of the 

 sulispecies of jxtUidpfidm as far west as Texas, New Mexico, and 

 Colorado and the existence of an allied species, F. mold, in Utah and 

 Arizona are by no means inconsistent with a southwestern origin. 

 There seems to have been some obstacle to the spread of many forms 

 westward from Colorado and New Mexico, for no forms of tufa or 

 sanguinca, or of the microgyna and pallidefulva series are known to 

 occur in California. 



If we assume that the genus Formica had its origin in a southwestern 

 center, we must conclude that the emigration of species from this 

 region to other parts of North America and especially to Asia over a 

 Bering Sea land-bridge and to Europe across Scharff's Greenland- 

 Iceland land-bridge, has extended over a very long period of time. 

 The first emigrants must have reached the Old World before Oligo- 

 cene and probably as early as late Mesozoic times, because we find 

 F. flori as a common ant in the Baltic amber. Precursors of the rufa, 

 sanguinea, and exsecta groups must have reached the Old World at 

 the same time or somewhat later. That these various species have 

 since occupied the territory which they invaded, without being dis- 

 lodged during the glacial epoch is very probable. Both Kolbe ^ 

 and Scharff have recently given good reasons for maintaining that 

 the biogeographical conclusions so generally accepted as following 

 from the statements of those geologists who have asserted the exis- 

 tence of a very extensive and severe glaciation of the northern por- 

 tions of all the great land masses in the northern hemisphere during 

 the Pleistocene, must be, to a considerable extent, erroneous. These 

 investigators hold that glaciation could not have been so extensive 

 as to have "sterilized" the greater part of North America and Eurasia, 

 but that temperature and other conditions during the Pleistocene must 

 have been sufficiently favorable to admit of the survival of a rather 

 considerable fauna and flora in the immediate neighborhood of the 

 glaciers. Hence many species were able to maintain the station 

 which they had occupied since early Tertiary or Mesozoic times. As 

 it is probable that these views will before long cause a revolution in 



> Glazialzeitliche reliktenfauna etc. Loc. cit. 



