sula of Michigan; thruout most of New England south of the Androscoggin River; in the southern third of Indiana and the adjacent areas of southern Illinois; in the Southeast wherever the northern faunas of the Appalachians come into contact with southern species at lower elevations, particularly in the eastern two-thirds of Kentucky and in the Cumberlands and the lower hill country of central Tennessee; and farther west in more limited areas that lie between practically every one of the cynipid faunas all the way to the Pacific Coast. I have similar hybrid series in my European collections of Cynips from more northern Denmark, the southernmost portion of Finland, from Bohemia, and from the upper Danube valley.
In some of these localities, as for instance in the neighborhood of our own laboratory at Bloomington, Indiana, the hybrid individuals may constitute 30 to 50 per cent of each collection. In places in the Cumberlands of Tennessee the hybrids may amount to 80 per cent or more of the cynipid populations. Whether the areas of transition among the Cynipidae are the same as those among other organisms must be determined by studies on these other groups. Nevertheless, if Jordan's Law holds as often as it would appear, species usually have close relatives in adjacent areas, whatever group of plants or animals they represent, and such close relatives are usually fertile inter se and should give rise to inter-specific hybrid individuals as often as we have found them among the Cynipidae.
But do such inter-specific hybrids ever give rise to populations that deserve to be called species?
It must be remembered that transition zone populations grade in every direction into the pure populations between which they are hybrid. Any portion of such a hybrid population is different biometrically from any other portion of that population. The genes available at one point in the transition zone are not equally available at every other point in the zone. There is no common heredity within the population. It does not satisfy our concept of a species (p. 20), no matter how extensive the area over which it occurs.
But if such a population, of hybrid origin, should in some way become isolated, then it might in the course of time become a fairly uniform population. Relieved from the continual introduction of genes from the parental stocks, the hy-
