In some cases, such as the Ethiopian and Neotropical and the Palaearctic and Nearctic regions, the faunas, although distinct, are related, several forms on opposite sides of the Atlantic being analogous, e.g.
The Tracheophonae among the Passeriformes, the possessors of this specialized although low type of syrinx, form a tolerably well-marked group, entirely neotropical.
Sclater' was the first to divide the world into a few great " regions," the Palaearctic, Ethiopian, Indian and Australian forming one group, the " Old World " (Palaeogaea); and the Nearctic and Neotropical forming a second, the New World (Neogaea).
They express the main complexes of land with their dependencies in well-chosen terms; for instance the " Neotropical region " stands short for South and Central America with the Antilles.
The differences between the Neotropical avifauna and that of North America are fundamental and prove the independence or superior value of the Neotropical region as one of the principal realms.