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 a 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, — lstly, 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 ; 



