FIELD ZOOLOGY AND GEOGRAPHY 339 



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 distribution 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 north-eastern 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, present and past, of the world since a period geologic- 

 ally 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 distribution 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 accordance with the 

 views of the extremists of this school we could, from the exclusive 

 study of certain groups of animals, conclude that at different 

 periods the United States and almost every other portion of the 

 earth were connected by land and severed from all other regions by 

 water — and, from the study of certain other groups of animals, 

 arrive at directly opposite and incompatible conclusions. 



The most brilliant and unsafe exponent of this school was 

 Ameghino, who possessed and abused two gifts, both essential to 

 the highest type of scientist, and both mischievous unless this 

 scientist possess a rare and accurate habit of thought joined to 

 industry and mastery of detail — namely, the gift of clear and 



