314 TROPICAL NATURE, AND OTHER ESSAYS. 



This general conclusion is of great importance in the 

 study of the geographical distribution of animals, be- 

 cause it bids us avoid the too hasty assumption that the 

 countless anomalies we meet with are to be explained 

 by great changes in the distribution of land and sea, 

 and leads us to rely more on the inherent powers of 

 dispersal which all organisms possess, and on the union 

 or disruption, extension or diminution, of existing lands 

 but always in such directions and to such a limited 

 extent as not to involve the elevation of what are now 

 the profoundest depths of the great oceans. 



Zoological Regions. We will now proceed to sketch 

 out the zoological features of the six great biological 

 regions ; and will afterwards discuss their probable 

 changes during the more recent geographical periods, in 

 accordance with the principles here laid down. 



The Palcearctic Region. 



The Palaearctic, or North Temperate region of the 

 Old World, is not only by far the most extensive of the 

 zoological regions, but is the one which agrees least 

 with our ordinary geographical divisions. It includes 

 the whole of Europe, by far the largest part of Asia, 

 and a considerable tract of North Africa ; yet over the 

 whole of this vast area there prevails a unity of the 

 forms of animal life which renders any primary sub- 

 division of it impossible, and even secondary divisions 

 difficult. But besides being the largest of the great 

 zoological regions, there are good reasons for believing 

 this to represent the most ancient, and therefore the 

 most important centre of the development of the higher 



