342 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



(the Holarctic region) and migrated outward, radially, into South 

 America, Africa, and the Indo-Australian region. On a globe, or a 

 north-polar projection of the world, such evolution and migration is 

 easy to visualize ; Eurasia and North America, which have frequently 

 been connected, form a preponderating land unit, from which the 

 other continents radiate. I think no serious zoogeographer can dis- 

 agree radically \vith Matthew's view that secular climatic change has 

 had an enormous effect on aiihnal distribution, but it is questionable 

 that cHmate has directly influenced evolution itself.^ In regard to 

 his postulation of a northern origin of the faunas of South America 

 and other southern continents, however, it should be observed that 

 von Ihering (1907), Eigenmann (1909), and other eminent men have 

 come to a directly opposed conclusion, which emphasizes the relation- 

 ships of certain African and South American animals that they believe 

 did not originate in the north, and which, therefore, would demand 

 some sort of South Atlantic land bridge to account for their evident 

 community of origin. The necessity for a South Atlantic bridge has 

 also been strongly advocated by so eminent an authority as Regan 

 (1922), whose thoroughly sound data on the ostariophysan fishes are 

 of the greatest importance. 



In reviewing the distribution of American continental faunas in 

 connection with some recent studies of West Indian fresh-water 

 fishes, I have been struck with the amount of misinformation that 

 passes as sound ichthyological evidence among zoogeographcrs. Part 

 of this is the fault of the ichthyologists themselves. Papers on fish 

 distribution written by competent ichthyologists and based on 

 modern paleontological and ichthyological data are scarce, and the 

 distributional information in two recent general textbooks on fishes 

 (Kyle, MacFarlane) is scarcely to be relied on. On the other hand, 

 the recent dependable papers that do exist have been neglected by 

 most students of zoogeography, I have come across no modern 

 paper that summarizes the broader geographical aspects of the fresh- 

 water fishes of North and South America, and, in the belief that such 

 may be of some general interest, I shall attempt to supply this want 

 in very brief form. At the same time I shall speak about some of 

 the miplications of fresh-water fish distribution in the controversial 

 question of West Indian paleogeography, but, in compliance with my 

 thesis that biogeograpliical problems cannot be solved on the evidence 

 of one group, I shall not pontify on matters I know I am not competent 

 to settle. 



WHAT ARE FRESH-WATER FISHES? 



The importance of fresh-water fishes to students of geographical dis- 

 tribution depends primarily on two facts. Firstly, certain families of 

 fishes possess an ancient physiological inability to survive in salt sea 



* Except as one of the factors of natural selection. 



I 



