coast we were dealing with pezomachoides, which is another species. At first the population was a mixture of insects of erinacei and pezomachoides and hybrids between, and it was only near the ocean that we found pure colonies of this new species, altho the typically smooth gall of pezomachoides was dominant to the western limits of the influence of the insect. But pezomachoides, altho very similar to erinacei, varies within limits that are different from those of erinacei. We shall find pezomachoides wherever we collect, until some day we turn in from the coast toward the heart of Georgia and meet a larger form of naked gall which will be derivatus; and then when we cross the southern front of the Blue Ridge and go into the Cumberlands we shall find the insect called advena, and in Kentucky it will be ozark, and in southern Indiana it will be erinacei again. In each area we shall find a population obviously related to every other, but each will constitute a population whose limits of variation are different from those of any other. Sometimes these limits overlap, sometimes they are wholly within, sometimes they are wholly without the limits for each other species. Sometimes the populations are wholly segregated by mountain crests and divides, sometimes the populations hybridize and intergrade over broad zones of transition; but always each population is distinct in a great area which is the heart of the range of each species. This is the picture which has gradually developed over the 32,000 miles, thru the 12 years during which we have pursued species.
And now, one confusion needs explanation before we are finished with this part of our discussion. It must be pointed out that there is a biologic concept called species and a taxonomic category called species, and that the two are not always synonymous. The concept we have developed is the biologic concept to which all except the taxonomists must refer whenever they consider the problem of species. This is the sense in which even taxonomists, including ourselves, intend the word when it is used in most biologic connections. But this biologic species is, unfortunately, the first of the taxonomic categories, the fundamental unit with taxonomic significance. As a category taxonomists may label this a geographic variant, a variety, a subspecies, or a species. There
