CHAP. IV.] ZOOLOGICAL REGIONS. 67 



explained in Chapter IX. As a matter of convenience, and for 

 otlier 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 Palcearctic and termin- 

 ates with the Nearctic region. 



Objections to the system of CirciLiiipolar 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 " laiv of the distribution of 

 life in circumsolar 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 tliat these adaptive 

 peculiarities were suitable foundations for a classification, — 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 circum2Jolar distribution " leads to the division of 



F 2 



