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 0, ec, and d, on the other, 
and then, by another division, make 8, an area of the second, 
and ¢, 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 land 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 uumber 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 
