Page:The Gall Wasp Genus Cynips.pdf/59

 the Atlantic seaboard, retaining its long wings everywhere except in the most remote corner of its range, in the very southeastern corner of the United States, where it is now developing short-winged species.

There are a few further aspects of the distribution of Cynips which have some bearing on the problem of species.

1. The maps of the several species show that ranges vary from relatively small (e.g. the San Bernardino area of California and the Apache Trail area of Arizona) to very large (e.g. the whole of Central Europe or the northeastern quarter of the United States), but they more often involve areas of considerable size. This means that species are relatively stable entities which are not easily disturbed by mutations or immigrations of new stocks. If such new stocks are not submerged, interbreeding must proceed fast enough over these areas to maintain the uniformity of the species. It is difficult to conceive how this can occur in the largest of the areas.

2. The ranges of these species are more often large in the regions which are topographically most uniform. The relatively rugged Central European area is an exception to this rule. Nevertheless, the general condition indicates that the isolation afforded by topographically diverse areas may be a considerable factor in favoring the multiplication of species.

3. There are, on the other hand, areas of relatively uniform topography within which distinct species have been isolated. This is true of the Canadian, Coastal Plain, northern Middle West, southern Middle West, Ozark, east Texas, and Central Texas areas in the United States. None of these areas appears to be bounded in such a way as to furnish enough geographic factors to isolate species. Distance must be an isolating factor in the origin and propagation of species. This item is never listed among factors of distribution, but I am inclined to believe that in an area like the eastern United States it is one of the most important factors. Distance would act as a barrier by preventing an interchange of genes thruout a large population, thus favoring the development of local races.

4. Another fact showing the importance of isolation is the common occurrence of hybrid individuals in transition areas between species. These hybrid populations are so extensive in the more uniform, eastern two-thirds of the United