Page:The Gall Wasp Genus Cynips.pdf/77

 of the insects of the Coast Ranges have northern varieties that reach their southern limits and southern varieties that reach their northern limits near San Francisco Bay. If the migration in such cases had been wholly from the north or wholly from the south, it is not easy to understand why the break should occur near San Francisco Bay; but the situation is explainable if it is presumed that both stocks originated from the eastern Sierras, that the northern variety reached San Francisco Bay from the north and the southern variety from the south, and that the Bay never was crossed until a geologically recent day.

The Eastern American group of subgenera of Cynips was differentiated in the late Miocene or early Pliocene, as the following considerations may show.

None of these subgenera are represented west of the Great Basin today, and it is probable that they were prevented from reaching the Sierras by the development of the Great Basin deserts in the Miocene.

That the subgenera were distinct and most if not all of the present-day species differentiated before the end of the Pliocene is attested by the fact that all but one of the species found east of the Great Plains is represented by close relatives, either very closely related species or varieties of the same species, in the Rocky Mountain area. The following table summarizes the situation.

This eastern extension of the Rocky Mountain fauna must have occurred before the Rockies reached their heights and thus caused the aridity of the Great Plains in the Pliocene. Today, between the easternmost oaks in Colorado and the westernmost extension of oaks in Kansas there are three or