The Palaearctic and the Nearctic. But to justify such a division it is necessary to establish either an exclusive possession or a marked predominance of types in the one which are correspondingly deficient in the other.
Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated.
They are six in number: (1) Palaearctic, including Europe, Asia north of the Himalaya, and Africa north of the Sahara; (2) Ethiopian, consisting of Africa south of the Atlas range, and Madagascar; (3) Oriental, including India, Indo-China and the Malay Archipelago north of Wallace's line, which runs between Bali and Lombok; (4) Australian, including Australia, New Zealand, New Guinea and Pol y nesia; (5) Nearctic or North America, north of Mexico; and (6) Neotropical or South America.
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.
These facts have led some naturalists to include the Palaearctic and Nearctic regions in one, termed Holarctic, and to suggest transitional regions, such as the Sonoran, between North and South America, and the Mediterranean, between Europe and Africa, or to create sub-regions, such as Madagascar and New Zealand.