That branch of zoölogical science which treats of the geographical distribution of animals is known as zoögeography. None of the zoölogical sciences has contributed more to a knowledge of the facts with which zoögeography deals than the science of entomology. Various divisions of the surface of the earth, based upon the character of the living beings which inhabit them, have been suggested. At the present time, however, it is agreed that in a general way five major subdivisions are sufficient for the purposes of the science, and we therefore recognize five faunal regions, namely, the _Palæarctic_, which includes the temperate regions of the eastern hemisphere; the _Indo-Malayan_, covering the tropics of Asia and the islands lying south of that great continent, including Australia; the _Ethiopian_, covering the continent of Africa south of the lands bordering on the Mediterranean, and extending northward into the southern part of Arabia; the _Neotropical_, covering the continent of South America and the islands of the Caribbean Sea and the Gulf of Mexico; and, finally, the _Nearctic_, covering the temperate and polar regions of North America. The butterflies with which this volume deals are mostly nearctic species, only a few species representing the neotropical region being found as stragglers into the extreme southern portion of the United States. These five faunal regions are characterized by the presence of certain groups of insects which are more or less peculiar to them. In the Palæarctic Region, for instance, we find a very great development of the _Satyrinæ_, of the genera _Argynnis_, _Melitæa_, and _Lycæna_, and of the genus _Colias_. The genus _Papilio_ is but poorly represented, there being only three species found on the entire continent of Europe, and comparatively few in Asia north of the Himalayan mountain-ranges.