Moths and Butterflies of the United States, East of the Rocky Mountains: With Over 400 Photographic Illustrations in the Text and Many Transfers of Species From Life; The Butterflies. E-book. Formato PDF Sherman Foote Denton   -  Forgotten Books, 2017  - 

The hard and fast lines fixed by naturalists do not really exist in Nature. As daylight blends into darkness and night again into day, or as the colors of the rainbow softly shade into one another, the violet into the blue, and the blue into the green, etc., so the different groups of insects pass by almost insensible gradations one into another.