CHAP. IV. | ZOOLOGICAL REGIONS. 53 
of the most important and interesting phenomena which a 
study of the distribution of animals presents to us. 
The merit of initiating a more natural system, that of determin- 
ing zoological regions, not by any arbitrary or @ priori considera- 
tion but by studying the actual ranges of the more important 
groups of animals, is due to Mr. Sclater, who, in 1857, established 
six primary zoological regions from a detailed examination of 
the distribution of the chief genera and families of Birds. 
Before stating what these regions are, what objections have 
been made to them, what other divisions have been since 
proposed, and what are those which we shall adopt in this 
work, it will be well to consider the general principles which 
should guide us in the choice between rival systems. 
Principles on which Zoological Regions should be formed. 
It will be evident in the first place that nothing like a per- 
fect zoological division of the earth is possible. The causes 
that have led to the present distribution of animal life are so 
varied, their action and reaction have been so complex, that 
anomalies and irregularities are sure to exist which will mar the 
symmetry of any rigid system. On two main points every sys- 
tem yet proposed, or that probably can be proposed, is open to 
objection; they are,—Istly, that the several regions are not-of 
equal rank ;—2ndly, that they are not equally applicable to all 
classes of animals. As to the first objection, it will be found 
impossible to form any three or more regions, each of which differs 
from the rest in an equal degree or in the same manner. One 
will surpass all others in the possession of peculiar families ; 
another will have many characteristic genera; while a third will 
be mainly distinguished by negative characters. There will also 
be found many intermediate districts, which possess some of the 
characteristics of two well-marked regions, with a few special 
features of their own, or perhaps with none; and it will be a 
difficult question to decide in all cases which region should 
possess this doubtful territory, or whether it should be formed 
into a primary region itself. Again, two regions which have 
now well-marked points of difference, may be shown to have been 
much more alike at a comparatively recent geological epoch ; 
