The geographical distribution of animals, with a study of the relations ...

6;chap, iv.] ZOOLOGICAL REGIONS. 57of its inhabitants, by which the present state of things hasbeen brought about. For this purpose we require a groupwhich shall be dependent for its means of dispersal on the distributionof land and water, and on the presence or absenceof lofty mountains, desert plains or plateaux, and great forestssince these are the chief physical features of the earth's surfacewhose modifications at successive periods we wish to discover.It is also essential that they should not be subject to dispersalby many accidental causes ; as this would inevitably in timetend to obliterate the effect of natural barriers, and produce ascattered distribution, the causes of which we could only guess at.Again, it is necessary that they should be so highly organized asnot to be absolutely dependent on other groups of animals, andwith so much power of adaptation as to be able to exist in oneform or another over the whole globe. And lastly, it is highlyimportant that the whole group should be pretty well known,and that a fairly natural classification, especially of its minordivisions such asfamilies and genera, should have been arrivedat ; the reason for which last proviso is explained in our nextchapter, on classification.Now in every one of these points the mammalia arepreeminent; and they possess the additional advantage of being themost highly developed class of organized beings, and that towhich we ourselves belong. We should therefore construct ourtypical or standard Zoological Regions in the first place, from aconsideration of the distribution of mammalia, only bringing toour aid the distribution of other groups to determine doubtfulpoints. Regions so established will be most closely in accordancewith those long-enduring features of physical geography,on which the distribution of all forms of life fundamentallydepend ; and all discrepancies in the distribution of otherclasses of animals must be capable of being explained, eitherby their exceptional means of dispersion or by special conditionsaffecting their perpetuation and increase in each locality.If these considerations are well founded, the objections ofthose who study insects or molluscs, for example,— that ourregions are not true for their departments of nature—cannot beVol. I.—