10 INTRODUCTION 



proportioned to the nature of the barrier which sepa- 

 rates the two regions. Transverse mountain chains, 

 such as the Himalayas or Atlas, separate two different 

 faunas. Similarly, lands separated by a broad and 

 deep belt of ocean may have quite different faunas ; 

 those separated by a shallow and narrow strait may 

 have almost identical animals. Such facts led him to 

 believe that each species originated in a particular 

 region, and spread out from that region till it was 

 stopped by some barrier to further distribution. The 

 remarkable features of the animals of isolated regions 

 can be explained on this hypothesis as the result of 

 descent with modification from the original forms. 



One result of the direction thus given to the study of 

 distribution has been to make the division of the globe 

 into zoogeographical regions, and the study of the faunas 

 of the different regions, of supreme importance, and it 

 is this aspect which is studied in most books on the 

 subject. But, obviously, another and more truly 

 geographical point of view is possible. We may study 

 the animals of the natural regions of the globe as form- 

 ing a part of the features of those regions, and in this 

 case attention is directed rather to the adaptations 

 displayed by the animals, than to their zoological 

 relationships. 



In deliminating the natural regions of the globe the 

 botanists have played a large part, for they have shown 

 that the existence of the great plant associations 

 depends upon the climate, the relief, and so forth of 

 the region. To the practised eye a collection of plants 

 from e. g. the Mediterranean region gives a great deal 

 of information in regard to the region. For example, 

 the small silvery leaves of such plants, and the other 

 devices for husbanding water, together with the occur- 



