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

chap, iv.] ZOOLOGICAL REGIONS. 55developed and adjusted, isthe living barrier that now keeps outintruders.When it is ascertained that the chief differences whichnow obtain between two areas did not exist in Miocene orPliocene times, the fact is one of great interest, and enables usto speculate with some degree of probability as to the causes thathave brought about the present state of things ; but it is not areason for uniting these two areas into one region. Our objectis to represent as nearly as possible the main features of thedistribution of existing animals, not those of any or all pastgeological epochs. Should we ever obtain sufficient informationas to the geography and biology of the earth at past epochs, wemight indeed determine approximately what were thePlioceneor Miocene or Eocene zoological regions ; but any attempt toexhibit all these in combination with those of our own period,must lead to confusion.The binary or dichotomous system, although it brings outthe fundamental differences of the respective regions, is aninconvenient one in its application, and rather increases thanobviates, the difficulty as to equality or inequality of regions;for although a, b, c, and d, may be areas of unequal zoologicalrank, a being the most important, and d the least, yet thisinequality will probably be still greater if we first dividethem into a, on one side, and b, c, and d, on the other,and then, by another division, make b, an area of the second,and c, and d, of the third rank only.Coming to the second objection, the often incompatibledistribution of different groups of animals, affords ground foropposition to any proposed scheme of zoological regions.Thereis first the radical difference between laud and sea animals;the most complete barriers to the dispersal of the one, sometimesoffering the greatest facilities for the emigration of the other,and vice versa. A large number of marine animals, however,frequent shallow water only ;and these, keeping near the coasts,will agree generally in their distribution with those inhabitingthe land. But among land animals themselves there are verygreat differences of distribution, due to certain specialities