MOUNTAIN ANTS OF NORTH AMERICA. 499 



postjjlacial. If we may judge from the Florissant deposits, the North 

 American ant-Hfe of Miocene times was poorer in si)ecies than that of 

 Europe, but we must bear in mind that the Florissant locality is a 

 very limited and elevated area and that the preserved ants are nearly 

 all males and females which happened to fall into a small lake during 

 their nuptial flight and after sinking to the bottom became embedded 

 in volcanic sediment. Many of the species became extinct, but some 

 of them, notably those of the genera Aphaeuogaster, La^'ius and Liome- 

 topum, were undoubtedly very closely related to forms of the same 

 genera still living in Colorado. To the descendants of this Miocene 

 fauna there was added during the Pleiocene a number of forms by 

 immigration from Asia over the Siberian-Alaskan land-bridge, proba- 

 bly at the same time and by the same route as the Strepsicerine and 

 Hippotragine antelopes. Probably, too, certain North American 

 ants passed into Asia at the same time, just as seems to have been 

 the case with the camels. Emery has recorded the occurrence of 

 Camponotus hcrculeanus subsp. pcnnsylvanwiis in Siberia and Burmah, 

 and the closely related C. japonicus was originally described by Mayr 

 as a mere variety of that subspecies. Probably such species as 

 Aphaenogaster subterranea, Myrmica lohicornis, Leptothorax muscorum, 

 some form of the subgenus Neomyrma closely related to the Eurasian 

 ruhida and the ancestor of A^. mutica, hunteri and aJdrichi, together 

 with several species of Formica, notably cinerea, rufibarbis, rufa, trun- 

 clcola and Polyergus rufescens, first entered North America during the 

 Pleiocene. Even at the present time few of these species have suc- 

 ceeded in extending their range to the Atlantic States. The origin of 

 the peculiarly eastern forms is more obscure. Probably a number of 

 them are Mesozoic and early Tertiary survivors, notably the Pone- 

 rinae, and the species of Strumigcnys, Myrmccina and Aphaenogaster. 

 Others may have come from Europe over the North Atlantic land- 

 bridge during the late Tertiary and have given rise to such forms as 

 Formica ulkei, exsectoides, and pallidefulva and Camponotus castaneus. 

 Some of these have migrated westward as far as the easternmost 

 ranges of the Rocky Mountains but none has reached the Pacific Coast 

 Eurythermal ubiquists like Tapinoma sessile, Prenolepis imparis and 

 Formica fusca may have existed in North America since the Oligocene 

 or Eocene. F. fusca and P. imparis are, as I have shown (1914), 

 almost identical with F. flori and P. henschei of the Baltic amber. 

 That elements derived from such various sources and migrations, 

 probably separated by long periods of time, should have gradually 

 evolved a number of subspecies and varieties in the localities to which 



