The several regions do not differ greatly in the number of families found in each. The Nearctic has 19, the Palæarctic 21, the Ethiopian 23, the Oriental 28, the Australian 29, and the Neotropical 23. But many of these families are only represented by a few species, or in limited districts; and if we count only those families which are tolerably well represented, and help to form the ornithological character of the region, the richness of the several tropical regions will appear to be (as it really is) comparatively much greater. The families that are confined to single regions are not very numerous, except in the case of the Neotropical region, which has 5. The Australian has only { 301}3, the Oriental 1, the Ethiopian 1, and the other regions have no peculiar families.
The distribution of the Passeres may be advantageously considered as divided into the five series of Turdoid, Tanagroid, Sturnoid, Formicarioid, and Anomalous Passeres. The Turdoid Passeres, consisting of the first 23 families, are especially characteristic of the Old World, none being found exclusively in America, and only two or three being at all abundant there. The Tanagroid Passeres (Families 24-33) are very characteristic of the New World, five being confined to it, and three others being quite as abundant there as in the Old World; while there is not a single exclusively Old World family in the series, except the Drepanididæ confined to the Sandwich Islands. The Sturnoid Passeres (Families 34-38) are all exclusively Old World, except that two larks inhabit parts of North America, and a few pipits South America. The Formicarioid Passeres (Families 39-48) are strikingly characteristic of the New World, to which seven of the families exclusively belong; the two Old World groups being small, and with a very restricted distribution. The Anomalous Passeres (Families 49-50) are confined to Australia.
The most remarkable feature in the geographical distribution of the Passeres is the richness of the American continent, and the large development of characteristic types that occurs there. The fact that America possesses 14 altogether peculiar families, while no less than 23 Old-World families are entirely absent from it, plainly indicates, that, if this division does not represent the most ancient and radical separation of the land surface of the globe, it must still be one of very great antiquity, and have modified in a very marked way the distribution of all living things. Not less remarkable is the richness in specific forms of the 13 peculiar American families. These contain no less than 1,570 species, leaving only about 500 American species in the 13 other Passerine families represented in the New World. If we make a deduction for those Nearctic species which occur only north of Panama, we may estimate the truly Neotropical species of Passerine birds at 1,900, which is almost exactly { 302}_one-third_ of the total number of Passeres; a wonderful illustration of the Ornithological riches of South America.