CHAP. IV. ] ZOOLOGICAL REGIONS. 57 
of its inhabitants, by which the present state of things has 
been brought about. For this purpose we require a group 
which shall be dependent for its means of dispersal on the dis- 
tribution of land and water, and on the presence or absence 
of lofty mountains, desert plains or plateaux, and great forests ; 
since these are the chief physical features of the earth’s surface 
whose modifications at successive periods we wish to discover. 
It is also essential that they should not be subject to dispersal 
by many accidental causes; as this would inevitably in time 
tend to obliterate the effect of natural barriers, and produce a 
scattered distribution, the causes of which we could only guess at. 
Again, it is necessary that they should be so highly organized as 
not to be absolutely dependent on other groups of animals, and 
with so much power of adaptation as to be able to exist in one 
form or another over the whole globe. And lastly, it is highly 
important that the whole group should be pretty well known, 
and that a fairly natural classification, especially of its minor 
divisions such as families and genera, should have been arrived 
at; the reason for which last proviso is explained in our next 
chapter, on classification. 
Now in every one of these points the mammalia are preemi- 
nent ; and they possess the additional advantage of being the 
most highly developed class of organized beings, and that to 
which we ourselves belong. We should therefore construct our 
typical or standard Zoological Regions in the first place, from a 
consideration of the distribution of mammalia, only bringing to 
our aid the distribution of other groups to determine doubtful 
points. Regions so established will be most closely in accord- 
ance with those long-enduring features of physical geography, 
on which the distribution of all forms of life fundamentally 
depend; and all discrepancies in the distribution of other 
classes of animals must be capable of being explained, either 
by their exceptional means of dispersion or by special condi- 
tions affecting their perpetuation and increase in each locality. 
If these considerations are well founded, the objections of 
those who study insects or molluscs, for example,—that our 
regions are not true for their departments of nature—cannot be 
Vou. L—6 
