ing many chapters to the
possible physical causes and modes of dispersal, he divided the land
into 21 realms which he shortly characterises, e.g. Australia as the
only country inhabited by marsupials, monotremes and meliphagous birds.
Ten main marine divisions were diagnosed in a similar way. Although some
of these realms were not badly selected from the point of view of being
applicable to more than one class of animals, they were obviously too
numerous for general purposes, and this drawback was overcome, in 1857,
by P.L. Sclater. ("On the general Geographical Distribution of the
members of the class Aves", "Proc. Linn. Soc." (Zoology II. 1858, pages
130-145.)) Starting with the idea, that "each species must have been
created within and over the geographical area, which it now occupies,"
he concluded "that the most natural primary ontological divisions of the
Earth's surface" were those six regions, which since their adoption
by Wallace in his epoch-making work, have become classical. Broadly
speaking, these six regions are equivalent to the great masses of land;
they are convenient terms for geographical facts, especially since the
Palaearctic region expresses the unity of Europe with the bulk of Asia.
Sclater further brigaded the regions of the Old World as Palaeogaea and
the two Americas as Neogaea, a fundamental mistake, justifiable to a
certain extent only since he based his regions mainly upon the present
distribution of the Passerine birds.

Unfortunately these six regions are not of equal value. The Indian
countries and the Ethiopian region (Africa south of the Sahara) are
obviously nothing but the tropical, southern continuations or appendages
of one greater complex. Further, the great eastern mass of land is so
intimately connected with North America that this continent has much
more in common with Europe and Asia than with South America. Therefore,
instead of dividing the world longitudinally as Sclater had done,
Huxley, in 1868 ("On the classification and distribution of the
Alectoromorphae and Heteromorphae", "Proc. Zool. Soc." 1868, page
294.), gave weighty reasons for dividing it transversely. Accordingly
he established two primary divisions, Arctogaea or the North world in
a wider sense, comprising Sclater's Indian, African, Palaearctic and
Neartic regions; and Notogaea, the Southern world, which he divided
into (1) Austro-Columbia (an unfortunate substitute for the neotropical
region), (2) Australasia