MOUNTAIN ANTS OF NORTH AMERICA. 487 



In tliis list of 41 Nearctic forms 25 are specifically, 6 subspecifically 

 and two varietally identical with Palearctic forms. 



The results of the foregoing study of the Transition and Boreal 

 ant-fauna agree in the main with those derived from other animals and 

 of plants, and suggest the same problems as to the original source of the 

 North American ant-fauna, the meaning of the differences between its 

 western and eastern constituents and of the much greater richness of 

 the former in species, subspecies and varieties. An intensive study of 

 the geographical distribution of any circumscribed group of organisms 

 necessarily involves an appeal to general historical considerations, since 

 no group can be satisfactorily studied as an isolated unit. One is 

 compelled, therefore, to assume an attitude towards certain hypotheses 

 which have been gradually elaborated and are more or less firmly 

 supported by the researches of many workers on many diflFerent groups. 

 In assuming such an attitude one is inevitably more or less biased by 

 the particular group or groups with which one is most familiar. Before 

 considering the hypothetical centers of origin and the migrations of 

 the various existing categories of insects and especially of the ants, 

 it seems advisable to determine, if possible, the geological age of these 

 categories. This has been attempted in three different ways: first, 

 by a study of paleontology, second, by a study of present distribution 

 on the supposition that forms with a wide and especially with a wide 

 and discontinuous range are older than forms with a limited, continu- 

 ous range, and third, by a combination of both of these methods. It 

 is evident that the first method is of great importance, the second by 

 itself of comparatively little value and open to many objections, and 

 that the value of the third method depends largely on the paleonto- 

 logical facts to which it may be able to appeal. It is, however, the 

 most comprehensive method and owing to the incompleteness of the 

 paleontological record, the only one that can be resorted to in the study 

 of many groups of organisms at the present time. 



Our knowledge of fossil ants is rather limited but of great signifi- 

 cance. The earliest loiown species are those of the Baltic amber, of 

 Lower Oligocene age. Mayr (186S) and I (1914) have described 

 nearly a hundred of these belonging to no less than 43 genera, 19 of 

 which are extinct and 24 still extant, viz: Ectatomma (subgen. Rhyti- 

 doponera), Euponera (subgen. Trachymesopus), Platythyrea, Ponera, 

 Sima, Monomorium, Erebomyrma, Vollenhovia, Stenanima, Aphaeno- 

 gaster, Myrviica, Leptothorax, Dolichoderus (subgen. Hypocllnea), 

 Iridomyrmex, Liomctopum, Plagiolepis, Gesomyrmex, Dimorphomyrmex, 

 Oecophylla, Prenolepis, Lasius, Formica, Pseudolasius and Camponolus. 



