species. In one species which constitutes an exception to this generalization, mutations are involved, and these will be treated in a later section of this paper.
Finally, the limits of variation of any character prove to be strikingly uniform thruout the great populations which we propose to call species. Whenever we have taken a reasonably large sample from any point over the usually considerable range of a species, the biometric data have not proved fundamentally different from data for any other fair sample from any other point in the range. The case of Cynips erinacei will serve to illustrate our experience. Erinacei is not only the most variable Cynips but one of the most variable cynipids I have examined. Reference to the descriptions in the systematic portion of this study will show that every one of the few characters which distinguish these insects from the most closely related species vary between limits approached by some one or another of the related species. The galls present more apparent variation, showing every gradation from smooth, naked, spherical, monothalamous structures (fig. 312) to densely spiny, ellipsoidal, polythalamous galls (fig. 315) which may have as many as eight larval cells. An initial experience with these extreme types of galls would lead one to believe they represented distinct species, and so they have always been classified heretofore. Increased material, however, has shown that every extreme and every one of the intermediate characters occurs thruout the range of erinacei.
The range of this species extends about 1,300 miles east and west and 450 miles north and south. We have samples of erinacei from nearly a hundred localities fairly well distributed over this tremendous area of possibly 500,000 square miles. In every large sample, erinacei is as variable as we have described it, and yet, after all, it is everywhere uniform—uniform in its constancy of variation. Even erinacei is, then, the sort of population which we would call a species.
Erinacei may present an extreme case, but it is not fundamentally different from the thing which one finds everywhere in nature. It is moreover, the picture of species to which our knowledge of mutation and Mendelian hybridization would lead us, and our definition of species must become genetic if we take into account the similarities and the differences which we find within a species. The essential uniformity of most
