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 



