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 Eegions in the fii'st place, from a 

 consideration of the distribution of mammalia, only bringing to 

 our aid the distribution of other groups to determine doubtful 

 points. Eegions 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 



