224 THE POPULAR SCIENCE MONTHLY. 



it is probably in reality larger than as yet suspected. For even the 

 species acknowledged to occupy this relationship to each other ex- 

 hibit quite different degrees of resemblance. I say " relationship," 

 for we must reasonably conclude that this resemblance is not acci- 

 dental 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 faunae had become separated through 

 the physical action of the glacial epoch, showing that from the pres- 

 ent 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 Noctum, 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 differ- 

 ences 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 differ- 

 ences, 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 por- 

 tion 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 



