498 WHEELER. 



in preventing the development of an indigenous ant fauna. On the 

 other hand the Rocky Mountains occupy a very large area and were 

 much less affected by the unfavorable climate of the Ice Age. They 

 therefore remained as a preserve of Pleiocene species and during more 

 recent times became a center of origin of many races and varieties. 

 For evidence in support of this contention we have only to compare 

 the Nearctic and Palearctic forms of the genera Myrniica, Leptothorax, 

 Lasius, Formica, Poli/ergus and Camjponoius. 



I advanced the opinion that the Rocky Mountains were probably the 

 center of origin of the genus Formica in my paper on this group (1913), 

 but I now accept Handlirsch's view that not only the genus Formica, 

 but the whole family Formicidse had its origin during the Mesozoic 

 in Eurasia and believe, with Kolbe, that Central or Eastern Asia is, 

 as seems to be the case with so many other groups of organisms, the 

 most likely spot in which to seek the origin of the ants. The views 

 of Matthew and Clark on the development of the specialized forms 

 in the center of the geographical range of a group and the relegation 

 of the older and more primitive forms to the periphery of that range, 

 appear at first sight to support my former contention, but it now seems 

 to me to be more probable that the Rocky Mountains constitute only 

 a secondary, more recent center of speciation, and that they are much 

 more important as a region of conservation. It may be noted in this 

 connection that Japan is evidently a similar secondary center of 

 speciation for the same genera, since in that country the common 

 holarctic species of Myrmica, Lasius, Formica, Polyergus and Campo- 

 notus have developed several peculiar subspecies and varieties. The 

 same is true of the Eastern United States, which are also characterized 

 by the development of endemic forms of Myrmica, Formica, etc. 



This brings us, finally, to the problem with which we started, the 

 pronounced difference between the ant faunas of western and eastern 

 North America, a diflference very similar to what has been so often 

 noticed in other groups of organisms. It may be readily attributed 

 to a difference in survival after the glaciation of the northern portion 

 of the continent, since the ice-sheet is known to have advanced con- 

 siderably further south in the eastern than in the western half of the 

 continent, while the Gulf of Mexico formed an impassable barrier to a 

 directly southward emigration of species. Hence we should expect a 

 much more meager survival of species in the Eastern than in the 

 Western United States. The differences of character in the endemic 

 forms of the two regions, however, must be due to other conditions, 

 some of which were undoubtedly preglacial, while others were as clearly 



