Page:The Gall Wasp Genus Cynips.pdf/79

 supply a present-day means of eastern migration for only those species of Cynips that occur on the Gambelii-macrocarpa oaks. There are two such species, Cynips villosa and C. hirta. Villosa may, for all that is now apparent, have crossed from the more northern Rockies. On the other hand, the more southern concentration of the varieties of C. hirta, in part upon the chestnut oaks, suggests that this species came eastward by the Colorado-Missouri route which, we shall show in a moment, was followed by the remaining species of the genus. Any oak-inhabiting cynipid that crossed in this part of the Plains must have done so before the extermination of the oak flora in those areas in the Pliocene.

It is possible that during the southernmost extensions of the glaciers of the Pleistocene some increase in moisture allowed oak to return to some southern parts of the Great Plains. It is certain, however, that the several stocks of Cynips had come east before then, because in the northern Middle West there are several species which, as we have shown (page 59), seem to have had a hybrid origin in the Pleistocene. If the southern extension of the glaciers at that time crowded northern varieties into the ranges of southern varieties of the same species, with consequent hybridization of the close relatives, it follows that northern and southern varieties were already differentiated in the eastern United States.

Altho eastern and western species of oak make rare contacts in the Texas Panhandle and in northeastern New Mexico, the affinities of all the Cynipidae of Texas east of the Pecos River are clearly with those of the eastern United States, while all the Cynipidae of West Texas are of more direct origin from the Arizona-New Mexico stocks. The geologic record indicates that the desert boundary between West Texas and the more eastern part of the state is more ancient than the genus Quercus. One may conclude that the cynipid fauna of eastern and central Texas has been derived from the north and east and not directly from the Southwest.

The present-day concentration of the species of Acraspis (fig. 49) indicates that the eastern migration did not occur very far north of Texas. It probably occurred in Kansas and Missouri, or not far north or south of the boundaries of those states.