54 DISTRIBUTION OF ANIMALS. [PART I. 
and this, it may be said, proves their fundamental unity and 
that they ought to form but one primary region. To obviate 
some of these difficulties a binary or dichotomous division is 
sometimes proposed; that portion of the earth which differs 
most from the rest’ being cut off as a region equal in rank 
to all that remains, which is subjected again and again to 
the same process. 
To decide these various points it seems advisable that con- 
venience, intelligibility, and custom, should largely guide us. 
The first essential is, a broadly marked and easily remembered 
set of regions; which correspond, as nearly as truth to nature 
will allow, with the distribution of the most important groups 
of animals. What these groups are we shall presently explain. 
In determining the number, extent, and boundaries of these 
regions, we must be guided by a variety of indications, since 
the application of fixed rules is impossible. They should evi- 
dently be of a moderate number, corresponding as far as 
practicable with the great natural divisions of the globe marked 
out by nature, and which have always been recognized by 
geographers. There should be some approximation to equality 
of size, since there is reason to believe that a tolerably extensive 
area has been an essential condition for the development of 
most animal forms; and it is found that, other things being 
equal, the numbers, variety and importance of the forms of 
animal and vegetable life, do bear some approximate relation 
to extent of area. Although the possession of peculiar families 
or genera is the main character of a primary zoological region, 
yet the negative character of the absence of certain families 
or genera is of equal importance, when this absence does not 
manifestly depend on unsuitability to the support of the group, 
and especially when there is now no physical barrier preventing 
their entrance. This will become evident when we consider that 
the importance of the possession of a group by one region de- 
pends on its absence from the adjoining regions; and if there is 
now no barrier to its entrance, we may be sure that there has 
once been one ; and that the possession of the area by a distinct 
and well balanced set of organisms, which must have been slowly 
