opinion (1923:140) that “it is very doubtful if there was any possibility of an interchange of species of oak between the Old and New Worlds after late Eocene or Oligocene times,” and this may be the latest origin we may presume for the genus Cynips.
On the other hand, it is difficult to understand why both the primary subdivisions of the genus did not spread both to the east and to the west of their Southwestern center of origin if they migrated very much before the Great Basin became so arid as to interpose an effective barrier to further interchange of eastern and western species. It is understandable that each group might have begun its migration in a particular direction, but our knowledge of the present-day distribution of organisms would lead us to expect that a group should, in sufficient time, radiate in every direction in which there are no recognizable barriers to migration. But the rising mountains of the Pacific Coast probably did not bring about the development of the Great Basin deserts until the Miocene. If the Eurasian-Pacific-American branch of the genus crossed on to the Coast in the Miocene, its failure to radiate in other directions might be explained as due to the subsequent development of aridity in the Great Basin before the group had time to move back across that area. But if the group must be taken out of the Southwest and across Alaska at an earlier date, as Berry's statement would require, it is more difficult to understand why the Pacific Coast subgenera did not have an opportunity, before the Great Basin became arid, to spread back into the more eastern United States.
In the southernmost mountains of Arizona there is a living variety of the Pacific Coast species Cynips (Antron) guadaloupensis. This is the only representative of that subgenus known from east of the Sierras, but it is matched by a few cases in other cynipid genera and by a few Pacific slope trees, reptiles, and other organisms that have stray relatives in southern Arizona. These strays are, however, such close relatives of existent Californian species that they are probably to be interpreted as more recent arrivals in Arizona rather than remnants of the primitive stock before it moved westward into California. During the Quaternary the Great Basin had a more moist climate than it had had since the Miocene, and the area just north of the Gulf of California
