212 GENERAL PRINCIPLES OF ZOOLOGY. 



they were distributed in earlier times. Finally, by means 

 of anatomy and embryology we must have the conditions 

 of relationship of animals laid before us in a very detailed 

 manner. 



Difficulties of the Subject. The path to the solution 

 of the subject, here only briefly sketched, is well-nigh end- 

 less; what has hitherto been studied can only have the 

 importance of a preliminary proof that zoology, with its 

 prevailing views of the changes of animals and of the earth, 

 is on the right track. It would be a test of the correctness 

 of this view, if it could be shown that the faunal similarity 

 of two land areas depends, in the first place, upon how long 

 they have been in close connection with each other, con- 

 sequently allowing an interchange of the animals inhabit- 

 ing them. Two land areas, separated early in the earth's 

 history, and never again connected, must have very differ- 

 ent faunal characters; if two areas are still in connection, 

 or only recently separated, the faunal differences will be 

 relatively less. In carrying out more closely the points of 

 view mentioned, animal geographers have attempted to 

 mark off the great faunal belts of the earth, the animal 

 provinces or regions, and within these again less important 

 divisions, subregions. These provinces have been based 

 chiefly upon the manner of distribution of mammals, some- 

 what upon that of birds and the other animals ; for the man- 

 ner of distribution of mammals is chiefly determined by 

 those changes of the earth's surface which are best known 

 geologically and possess most interest. A rising or sinking 

 of the earth's surface often opposes impassable barriers to 

 most mammals : rising, because it leads to the formation of 

 mighty-glaciered mountain-chains; sinking, because as the 

 result arms of the sea are formed, which perhaps, even if 

 only narrow, interpose between two hitherto connected 

 land areas straits which are impassable for most mam- 

 mals. Birds and insects which fly well are less affected by 

 all such changes of the earth's surface; the majority of 

 them can fly over arms of the sea and mountain-chains, for 



