The study of the forms referable to the first or European element in the North American Moths, is complicated by the circumstance of the great range in the character and amount of the differences separating the related species now living so widely apart. These related species are found, in fact, to offer also difficulties as to their correct nomenclature. We have seen that the term “representative species,” like that of “prophetic types,” hides the real significance of the inter-resemblance which is that of a common descent. In naming these “representative” forms we must be guided by the rule that where the differences are such that the species would be considered distinct if members of a common fauna, a different specific title must be given them, but where these differences are slight, and what, under the same supposition, would be considered only varietal, the species must be considered as still identical, though separated so long in time and by so wide a space. The study of those forms which are practically identical is, as we have seen, further complicated by the difficulty of deciding as to whether they may have been introduced since the discovery of the New World by the Spaniards, or whether the species have continued unaltered since the Tertiary in both America and Europe. While certain species such as Scoliopteryx are undoubtedly in this latter case, it is less easy to feel so sure with regard to this in instances like Dipterygia Pinastri and the various identical forms of Agrotis, but on the whole I am inclined to believe that the number of species artificially exchanged is very small, and that these identical species have simply proved more constant and are of an older type than the rest.