But even in single species of butterflies immune through unpalatability there is usually a great abundance of individuals. Thus _Danais chrysippus_, which is distributed over the whole of Africa, is a very common butterfly wherever it can live at all; and in North America, in which country there are only two widely distributed species of _Danais_, these often occur in enormous numbers. The beautiful large _Danais erippus_ Cramer (Pl. I, Fig. 8), is distributed over almost all America, and in many places is not only frequent, but occurs in great swarms. Usually it peoples the broad, open stretches of the western prairies of the United States, but when violent winds blow, as they do there in September especially, the insects are driven together into the small wooded spots of the prairie, and then they cover the trees in incredibly large crowds, often so thickly that the leaves are entirely hidden, and the trees look brown instead of green. Millions of butterflies go to make up such swarms, which have been observed in many parts of the United States, even quite in the East, in New Jersey, and elsewhere. Considering this extraordinary abundance of the immune species, it is not surprising that its palatable copy, _Limenitis archippus_ (Pl. I, Fig. 9), should also be widely distributed in North America, and in many places it is not rare, but even abundant. The enormous majority of _Danais erippus_ will protect the species which resembles it so closely, even though it is not rare. Any doubt as to this being a case of mimicry disappears in face of the fact that, in Florida, there flies a second very similar but much darker brown North American _Danais_, and that it is accompanied there by an equally dark variety of _Limenitis archippus_ (_L. eros_).