350 APPENDIX A 



intermittently distributed southward along these continental masses 

 when there were no breaks in their southward connection, and inter- 

 mittently exchanged between them when they were connected in the 

 north; and he also upholds the view that from a common ancestral 

 form the same species has been often developed in entirely disconnected 

 localities when in these localities the conditions of environment were 

 the same. 



The opposite view is that there have been frequent connections 

 between the great land masses, alike in the tropics, in the south 

 temperate zone, and in the antarctic region. The upholders of this 

 theory base it almost exclusively on the distribution of living and fossil 

 forms of life; that is, it is based almost exclusively on biological and 

 not geological considerations. Unquestionably, the 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 distribu- 

 tion must be accounted for by the existence of some former direct land 

 bridge extending, for instance, 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 northeast- 

 ern Asia. The trouble is that as more groups of animals are studied 

 from the standpoint of this hypothesis the number of such land bridges 

 demanded to account for the existing facts of animal distribution 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, pres- 

 ent 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 rea- 

 soning, the land bridges could be made a hundred instead of merely 

 ten in number. The facts of distribution are in many cases inexpli- 

 cable with our present knowledge; yet if the existence of widely sep- 

 arated but closely allied forms is habitually to be explained in accor- 

 dance with the views of the extremists of this school we could, from the 

 exclusive study of certain groups of animals, conclude that at different 



