chap, iv.] ZOOLOGICAL REGIONS. 55 



developed and adjusted, is the living barrier that now keeps out 

 intruders. 



When it is ascertained that the chief differences which 

 now obtain between two areas did not exist in Miocene or 

 Pliocene times, the fact is one of great interest, and enables us 

 to speculate with some degree of probability as to the causes that 

 have brought about the present state of things ; but it is not a 

 reason for uniting these two areas into one region. Our object 

 is to represent as nearly as possible the main features of the 

 distribution of existing animals, not those of any or all past 

 geological epochs. Should we ever obtain sufficient information 

 as to the geography and biology of the earth at past epochs, we 

 might indeed determine approximately what were the Pliocene 

 or Miocene or Eocene zoological regions ; but any attempt to 

 exhibit all these in combination with those of our own period, 

 must lead to confusion. 



The binary or dichotomous system, although it brings out 

 the fundamental differences of the respective regions, is an 

 inconvenient one in its application, and rather increases than 

 obviates, the difficulty as to equality or inequality of regions; 

 for although a, b, c, and d, may be areas of unequal zoological 

 rank, a being the most important, and d the least, yet this 

 inequality will probably be still greater if we first divide 

 them 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 incompatible 

 distribution of different groups of animals, affords ground for 

 opposition to any proposed scheme of zoological regions. There 

 is first the radical difference between laud and sea animals; 

 the most complete barriers to the dispersal of the one, sometimes 

 offering 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 inhabiting 

 the land. But among land animals themselves there are very 

 great differences of distribution, due to certain specialities 



