OHAP. IV. ] ZOOLUGICAL REGIONS. 67 
explained in Chapter 1X. As a matter of convenience, and for 
other reasons adduced in the same chapter, the detailed exposi- 
tion of the geographical distribution of the animals of the several 
regions in Part III. commences with the Palearctic and termin- 
ates with the Nearctic region. 
Objections to the system of Circumpolar Zones.—Mr. Allen’s 
system of “realms” founded on climatic zones (given at 
p. 61), having recently appeared in an ornithological work 
of considerable detail and research, calls for a few remarks. 
The author continually refers to the “law of the distribution of 
life in cireumpolar zones,’ as if it were one generally accepted 
and that admits of no dispute. But this supposed “law” only 
applies to the smallest details of distribution—to the range and 
increasing or decreasing numbers of species as we pass from 
north to south, or the reverse ; while it has little bearing on the 
great features of zoological geography—the limitation of groups 
of genera and families to certain areas. It is analogous to 
the “law of adaptation” in the organisation of animals, by 
which members of various groups are suited for an aerial, an 
aquatic, a desert, or an arboreal life ; are herbivorous, carnivorous, 
or insectivorous; are fitted to live underground, or in fresh 
waters, or on polar ice. It was once thought that these adaptive 
peculiarities were suitable foundations for a classificatiopn,—that 
whales were fishes, and bats birds; and even to this day there 
are naturalists who cannot recognise the essential diversity 
of structure in such groups as swifts and swallows, sun-birds and 
humming-birds, under the superficial disguise caused by adap- 
tation to a similar mode of life. The application of Mr. Allen’s 
principle leads to equally erroneous results, as may be well seen 
by considering his separation of “the southern third of Aus- 
tralia” to unite it with New Zealand as one of his secondary 
zoological divisions. If there is one country in the world whose 
fauna is strictly homogeneous, that country is Australia; while 
New Guinea on the one hand, and New Zealand on the other, 
are as sharply differentiated from Australia as any adjacent parts 
of the same primary zoological division can possibly be. Yet 
the “law of cirewmpolar distribution” leads to the division of 
