The only absolutely cosmopolitan families of Mammalia are those which are aerial or marine; and this is one of the striking proofs that their distribution has been effected by natural causes, and that the permanence of barriers is one of the chief {176}agencies in the limitation of their range. Even among the aerial bats, however, only one family--the Vespertilionidæ--is truly cosmopolitan, the others having a more or less restricted range. Neither are the Cetacea necessarily cosmopolitan, most of the families being restricted either to warm or to cold seas; but one family, the dolphins (Delphinidæ), is truly so. This order however will not require further notice, as, being exclusively marine the groups do not enter into any of our terrestrial regions. The only other family of mammals that may be considered to be cosmopolitan, is the Muridæ (rats and mice); yet these are not entirely so, since none are known to be truly indigenous in any part of the Australian region except Australia itself. In the class of Birds, a number of families are cosmopolites, if we reckon as such all which are found in each region and sub-region; but several of these are so abundant in some parts, while they are so sparingly represented in others, that they cannot fairly be considered so. We shall confine that term therefore, to such as, there is reason to believe, inhabit every important sub-division of each region. Such are, among the Passerine birds the crows (Corvidæ), and swallows (Hirundinidæ); among the Picariæ the kingfishers (Alcedinidæ); among other Land birds the pigeons (Columbidæ), grouse and partridges (Tetraonidæ), hawks (Falconidæ), and owls (Strigidæ); among the Waders the rails (Rallidæ), snipes (Scolopacidæ), plovers (Charadriadæ), and herons (Ardeidæ); and among the Swimmers the ducks (Anatidæ), gulls (Laridæ), petrels (Procellariidæ), pelicans (Pelecanidæ), and grebes (Podicipidæ). In the class of Reptiles there are few absolutely cosmopolitan families, owing to the scarcity of members of this group in some insular sub-regions, such as New Zealand and the Pacific Islands. Those which are most nearly so are the Colubridæ among snakes, and the Scincidæ among lizards. There is no cosmopolitan family of Amphibia, the true frogs (Ranidæ) being the most widely distributed. Neither is any family of Freshwater Fishes cosmopolitan, the Siluridæ, which have the widest range, being confined {177}to warm regions, and becoming very scarce in the temperate zones. Among the Diurnal and Crepuscular Lepidoptera (butterflies and sphinges) the following families are cosmopolitan:--Satyridæ, Nymphalidæ, Lycænidæ, Pieridæ, Papilionidæ, Hesperidæ, Zygænidæ, and Sphingidæ. Of the Coleoptera almost all, except some of the small and obscure families, are cosmopolitan. Of the terrestrial Mollusca, the Helicidæ alone are true cosmopolites.