it is probably in reality larger than as yet suspected. For even the species acknowledged to occupy this relationship to each other exhibit quite different degrees of resemblance. I say “relationship,” for we must reasonably conclude that his resemblance is not accidental or meaningless. It must indicate that both the European and American species are derived from a common stock. More than two years ago, I suggested that the faunæ had become separated through the physical action of the glacial epoch, showing that from the present distribution of species, and their degree of resemblance, no other hypothesis would explain all the facts in the case so satisfactorily. For we have to account, first, for the occurrence of nearly-related forms in localities as widely separated, geographically, as Texas and Germany; and then, again, for the occurrence, within our territory, of nearly-related species on the Alpine summits of Mount Washington and the distant arctic regions. The method of probable distribution of these latter species, through the action of the retreating ice, I have explained in the American Naturalist for March of this year.
In studying specimens of these related American and European species of Noctuæ, some new light bearing upon the question of their differences is attempted to be thrown in the present paper. I have endeavored to localize these differences upon some portion of the insect. In all the cases I have been able to investigate, these differences are expressed on the upper surface of the body and wings, and principally on the upper surface of the front pair of wings. Here it is that the first variations, which now have grown into specific differences, were probably expressed. And the reason for this seems to be, that this portion of the body is that usually exposed to the light and the action of external influences. The moths, during the daytime, rest with the front-wings dependent over the hind-wings, and nearly covering the body. While in the American and European related species the differences which lead us to call them distinct “species” are located on the front-wings chiefly, the under surfaces of both wings remain exceedingly similar in the contrasted forms. This is the portion which in the day time (the period of inaction for moths) is applied to the surface against which the insect rests, and is entirely shielded from exposure. Take, for instance, the European Catocala fraxini and the American Catocala relicta. The ground-color of the American species above and below is a bright, clear white. Now, beneath, both species show this color, but the upper surface of the fore-wings, in the European species, is obscured by an evenly-distributed admixture of blackish scales, so that the wings appear of a uniform obscure gray. The hind-wings, in the European species, are crossed by a band of bluish scales; in the American, this band is white, while I have recently detected a slight powdered edging of blue scales, difficult to perceive, and apparently hitherto unnoticed. The principal difference in ornamentation is, again, to be found on the