364 Appendix A 



distribution of many forms of life, past and present, 

 offers problems which with our present paleontological 

 knowledge we are wholly unable to solve. If we consider 

 only the biological facts concerning some one group of 

 animals it is not only easy but inevitable to conclude that 

 its distribution must be accounted for by the existence 

 of some former direct land bridge extending, for in- 

 stance, between Patagonia and Australia, or between 

 Brazil and South Africa, or between the West Indies and 

 the Mediterranean, or between a part of the Andean 

 region and northeastern Asia. The trouble is that as 

 more groups of animals are studied from the standpoint 

 of this hypothesis the number of such land bridges de- 

 manded to account for the existing facts of animal dis- 

 tribution is constantly and indefinitely extended. A 

 recent book by one of the most learned advocates of this 

 hypothesis calls for at least ten such land bridges between 

 South America and all the other continents, present and 

 past, of the world since a period geologically not very 

 remote. These land bridges, moreover, must, many of 

 them, have been literally bridges; long, narrow tongues 

 of land thrust in every direction across the broad oceans. 

 According to this view the continental land masses have 

 been in a fairly fluid condition of instability. By parity 

 of reasoning, the land bridges could be made a hundred 

 instead of merely ten in number. The facts of distribu- 

 tion are in many cases inexplicable with our present 

 knowledge; yet if the existence of widely separated but 

 closely allied forms is habitually to be explained in ac- 

 cordance with the views of the extremists of this school 

 we could, from the exclusive study of certain groups of 



