The geographical distribution of animals, with a study of the relations ...

chap, iv.] ZOOLOGICAL REGIONS. 53of the most important and interesting phenomena which astudy of the distribution of animals presents to us.The merit of initiating a more natural system, that of determiningzoological regions, not by any arbitrary or a priori considerationbut by studying the actualranges of the more importantgroups of animals, is due to Mr. Sclater, who, in 1857, establishedsix primary zoological regions from a detailed examination ofthe distribution of the chief genera and families of Birds.Before stating what these regions are, what objections havebeen made to them, what other divisions have been sinceproposed, and what are those which we shall adopt in thiswork, it will be well to consider the general principles whichshould guide us in the choice between rival systems.Principles on which Zoological Regions should he formed.—It will be evident in the first place that nothing like a perfectzoological division of the earth is possible. The causesthat have led to the present distribution of animal life are sovaried, their action and reaction have been so complex, thatanomalies and irregularities are sure to exist which will mar thesymmetry of any rigid system. On two main points every systemyet proposed, or that probably can be proposed, is open toobjection ; they are,—lstly, that the several regions are not ofequal rank ;—2ndly, that they are not equally applicable toclasses of animals. As to the first objection, it will be foundimpossible to form any three or more regions, each of which differsfrom the rest in an equal degree or in the same manner. Onewill surpass all others in the possession of peculiar families;another will have many characteristic genera ;be mainly distinguished by negative characters.allwhile a third willThere will alsobe found many intermediate districts, which possess some of thecharacteristics of two well-marked regions, with a few specialfeatures of their own, or perhaps with none; and it will be adifficult question to decide in all cases which region shouldpossess this doubtful territory, or whether it should be formedinto a primary region itself. Again, two regions which havenow well-marked points of difference, may be shown to have beenmuch more alike at a comparatively recent geological epoch;