4 GEOGRAPHICAL DISTEIBUTION. 



pear, they yet have all their logical explanation. A quarter of a 

 century ago, when the doctrine of independent creation still held 

 sway over the minds of most naturalists, and when the organic 

 universe was reiiected in the eye of the investigator as an incon- 

 gruous agglomeration of disjointed parts, there was, indeed, no 

 necessity for specially accounting for the facts, since they were 

 conceived to be such by reason of a previous ordination. Now, 

 however, when the full value of the evolutionary process is recog- 

 nised, and animate nature has come to be looked upon as a con- 

 crete whole, bearing special relations to its numberless parts, each 

 individual fact seeks its own explanation, which explanation must 

 of necessity stand in direct harmony with some previously observed 

 fact. When, therefore, we seek to unravel the tangle of zooge- 

 ography, and to harmonise its apparent incongruities, we must at 

 the outset admit that distribution, such as it is, is the outcome of 

 definite interacting laws — laws which stand in relation to each 

 other as absolutely as they do in any other field of action — and not 

 a hap-hazard disposition, as some would lead us to suppose, setting 

 all enquiry at defiance. 



The naturalist who in the Western Hemisphere journeys south- 

 ward from the ice-covered fields of British America fails'to notice 

 any very sudden or marked alternation in the character of the 

 faunas that successively meet his view. New features are being 

 constantly added, and old ones eliminated, but the interchange is 

 effected so gradually that it becomes difficult to detei-mine the 

 limitations that properly define one fauna from another. The fur- 

 bearing animals of the far north send their representatives into 

 regions which border the habitats of the more exclusively tropical 

 species, or are succeeded by f onus which differ but little from them. 

 The skunk, many of whose associates are animals of a distinctively 

 Arctic character, finds its way into Mexico, and the ermine, which 

 penetrates to the farthest northern point reached by mammals 

 generally, still lingers on in some of the Southern United States. 

 The Arctic fox is succeeded by the equally abundant types of the 

 grey and the red fox ; and similarly, the polar bear is followed on 

 the one side of the continent by the grizzly, and on the other by 

 the black bear. Having descended into the middle temperate 

 regions, the traveller still finds about him mostly the forms with 

 which he has already become acquainted. But many of the more 



