56 
Indiana University Studies 
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 neighbor- 
hood of our own laboratory at Bloomington, Indiana, the hy- 
brid individuals may constitute 30 to 50 per cent of each col- 
lection. In places in the Cumberlands of Tennessee the hy- 
brids 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. Neverthe- 
less, 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-spe- 
cific hybrid individuals as often as we have found them among 
the Cynipidae. 
But do such inter-specific hybrids ever give rise to popula- 
tions 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 popu- 
lation 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 m 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 be- 
come a fairly uniform population. Relieved from the con- 
tinual introduction of genes from the parental stocks, the hy- 
