PROBLEMS OF GEOGRAPHICAL DISTRIBUTION. 761 



(d. ) Personally, I regard the most probable ancestral home of 

 animals as some region not far from the shore, and I picture the 

 relations as follows : 



Dry Land 



Original 

 Home? 



The more detailed Problems of Geographical Distribution. 



Leaving the general, and at present very 4 obscure, problem 

 of the evolution of faunas, let us briefly notice some of the 

 more detailed questions of distribution. We shall content 

 ourselves with stating (i) a few of the outstanding facts, (2) 

 the factors determining why some animals are here and 

 others there, and (3) the usually recognised zoo-geographical 

 regions. 



Some of the Outstanding Facts of Geographical Distribution. 



(a.) Widely separated countries may have an essentially 

 similar fauna. Thus, there is much in common between 

 Britain and Northern Japan, and there is so much agree- 

 ment between the North European (Palaearctic) and the 

 North American (Nearctic) fauna that many unite the two 

 regions in one (Holarctic). 



