Page:The Gall Wasp Genus Cynips.pdf/73

 Atrusca, and Acraspis, could have spread to the north in the Rocky Mountain area, to the south in Mexico, and to the east toward the Atlantic Coast. On the other hand, if the genus were assumed to have arisen in Europe or Asia, it is difficult to understand why the Eurasian affinities should have been maintained onto the Pacific Coast of North America, and a primary subdivision of the genus not effected until the group reached the Southern Rockies. However, we are completely ignorant of the present-day occurrence of the genus in most of Asia (on pp. 447 to 453 we show that the species reported from Japan do not belong to the genus), and further discoveries may throw light on Asiatic beginnings of the group; but from the available data we would presume the southwestern American origin for the genus and its initial differentiation in that area.

That the point of origin was not north of central Arizona or New Mexico is indicated by the occurrence of numerous short-winged species both north and east of the region, while not a single short-winged species of the group is to be found in the southern halves of those states. We have already shown that the short-winged species represent more specialized developments of the long-winged stocks. One short-winged and one long-winged species of Cynips is known from central Mexico, but our knowledge of the gall makers of that country is still insufficient to make it certain that our genus did not originate somewhere in northern Mexico.

It is interesting to find that Trelease (1924:34), considering the origin of the American oaks, states that “their primary center of distribution appears to have been what is now Arizona.” Trelease's conclusion is based in part upon the opinion that the widely distributed Cretaceous oaks have left no descendants in present-day groups, and that the existent white oaks of Europe and Asia, and both the white and black oaks of America have developed from the type represented by the European, Tertiary Quercus Palaeo-Ilex. This type is presumed to have reached America at some time previous to the Cenozoic. Berry's criticism (1923:139) of this conclusion seems to be based upon a mis-interpretation of Trelease's involved presentation of the argument, but the matter needs more data than seem to have been derived from the Cretaceous fossils that are often dubiously referred to Quercus.