In Canada the typical _Nephele_ is the only form representing the genus Satyrus, except that possibly in some localities _Alope_ or intergrades may appear; but if so, it is only occasionally. In New York and part of New England a belt of latitude is passed where in one section or other both these forms fly, besides an endless variety of intergrades. Finally, _Alope_ emerges in the south from this belt as the only form, and inhabits a broad zone, which ends about with the southern line of North Carolina and of Tennessee, but at the southwest flies in parts of Texas, and has become slightly modified when compared with the _Alope_ of the middle States. And to the west, somewhere between New York and Illinois, _Alope_ disappears, and a slightly changed form of _Nephele_ presents itself, and occupies the country to and on the eastern slopes of the Rocky Mountains. In some cases this cannot be distinguished from the typical _Nephele_, but as a whole, it has taken a departure, and has come to have differences in its larva and chrysalis. I call this form variety _Olympus_ (after the companion of the satyr Marsyas when the latter had his little difficulty with Apollo). The relationship between _Alope_ and _Nephele_ is in good degree paralleled by L. _Arthemis_ and _Proserpina_, the first of which occupies the northern half of the Continent, but is dimorphic with the other in a belt of latitude which passes through the northern States from Maine to Wisconsin. _Proserpina_ emerges from this belt on the south, and grades imperceptibly into _Ursula_, which last changes gradually till it has acquired a type, in Arizona, as different from that in which it manifests itself in Pennsylvania as the Texan _Alope_ is from _Alope_ of New York. This belt is nearly coterminous on both north and south with the belt of dimorphism in the Satyrids. It is worthy of note also that the dimorphism of P. _Turnus_ begins inside this belt. In this last-named species it has been supposed that the melanic form (confined to the female, _Glaucus_) first originated by accident, and was afterwards perpetuated and obtained an advantage over the yellow form, and finally in good degree supplanted it throughout its southern area, and that the existence of enemies had much to do with the suppression of one form, while their absence favored the other. What influence has gradually transmuted _Alope_ into _Nephele_ it is difficult to conjecture. It could not here be the presence or absence of enemies which has affected one or other form. And if it is climatic, what can there be in common between the climate of Canada and Illinois which encourages _Nephele_ and extinguishes _Alope_? In a second paper I shall speak of _Pegala_ and the Pacific species of this genus.