492 WHEELER. 



Baltic amber and of Florissant, like the plants of the same formations, 

 show A'ery clearly the gradual cooling of climate during the early and 

 middle Tertiary. In the latitude of Sweden, where the amber was 

 formed, the climate seems to have been subtropical as early as the 

 Lower Oligocene, since the ants belonging to boreal genera such as 

 Foriiiica, Lasius, Prenolepis s. sir. etc. constitute a dominant compo- 

 nent of the fauna, at least in indi\dduals. During the Miocene the 

 climate of Colorado, as indicated by the Florissant plants, resembled 

 that of the Gulf States at the present time. The ants perhaps indi- 

 cate a slightly cooler and dryer climate, not unlike that now prevail- 

 ing at low altitudes in Colorado or New Mexico. 



I am inclined to believe, with Scharff, that the extent of the south- 

 ward migration or displacement of organisms beyond the border of 

 the ice sheet during glacial times has been exaggerated by many 

 authors. Still there must have been some displacement and consid- 

 erable extinction. It is at any rate clear that owing to the absence 

 of such a complete barrier to southward migration as the Alps and the 

 Mediterranean, otir North American fauna suffered much less severely 

 (luring the Ice Age than that of Europe. Moreover our fauna has 

 been greatly enriched since the Pleistocene by a northward immigra- 

 tion of numerous neotropical species into the Southern United States 

 by way of Mexico and the West Indies. The neotropical immigrants 

 among ants belong to the Doryline genus Eciton, to several Ponerine 

 genera (Neojionera, Pseudoponera, Ectatonima, Leptogenys and Odonto- 

 machus), to several MjTmicine genera {Pseudomyrma, Cryptocerus, 

 Macromischa, Xenomyrmex, Xiphomyrmex, possibly Pogonoviyrmex 

 and especially to the fungus-growing tribe Attini {Atta, Acromyrmex, 

 Traehymyrmex and Cyp>hoiuyr)nex), to the Dolichoderine genera 

 Foretius, Dorymyrmex and Iridomyrmex and to the Camponotine 

 genera Brachymyrmex, Prenolepis (subgen. Nylanderia) and Campono- 

 tus (subgen. Myrmothrix, Myrmobrachys and Myrmamblys). Some 

 of these genera {Pseudoponera, Odontomachus, Leptogenys, Iridomyr- 

 mex) are common to paleotropical regions and at once suggest the 

 question as to whether they originally reached South America during 

 the Cretaceous by way of Antarctica from Australia or came from 

 Asia by way of North America or over other land-connections from 

 other parts of the Old World, and therefore involve a discussion of the 

 hypothetical southern land-bridges, which several recent writers, 

 notably H. von Ihering and Scharff, have been very actively construct- 

 ing in order to explain certain cases of wide and discontinuous distribu- 

 tion among organisms. So far as the Formicidae are concerned I 



