DISCONTINUOUS DISTRIBUTION 259 



mention among northern plants, the oak (Quercus) and the 

 plane (Platanus), among animals, the newt Spelerpes, the 

 snapping turtles (Chelidridae) and the fresh-water mussels 

 (Unionidae) . Many other northern animals and plants have 

 a similar disjointed range in the midst of an entirely alien 

 southern fauna. These, and the northern groups that are 

 altogether absent from Central America, but reappear much 

 further south, are, I think, part of a very ancient invasion of 

 South America. If it is true that the South American animals 

 and plants which have taken almost complete possession of 

 Central America, poured into the latter from the south in 

 early Pliocene times, surely the northern groups must be 

 immeasurably older ! In their discontinuous range and fre- 

 quent isolation on mountain tops they show every sign o^f 

 antiquity, and it seems likely that they are being crowded 

 out rapidly by their stronger southern rivals, which are better 

 fitted to support the present climatic conditions of this region. 

 Dr. Wallace * suggested, as I mentioned above, that the 

 northern faunistic affinity of South America which is so cha- 

 racteristic, especially of Chile, but which we find to some 

 extent all along the Pacific co,ast, is mainly due to the Glacial 

 Epoch. He fancied that a migration, aided by gales and hurri- 

 canes during successive Glacial Epochs, when the mountain 

 range of the Isthmus of Panama, if moderately increased in 

 height, might have become adapted for the passage of northern 

 forms, would explain all these extraordinary features of distri- 

 bution. Dr. Wallace thought the resemblance only consisted 

 in a few plants and insects. He had very little idea of the real 

 extent and character of the northern element that has actually 

 penetrated into Central and South America. It is unlikely 

 that he would have supported these views had he known 

 of the range of the newts, turtles, freshwater mussels and 

 other groups. Moreover, we also have evidence of ancient 

 migrations of mammals and other animals, as well as plants, 

 in an inverse direction from the south to the north. It is 

 perfectly certain, therefore, that long anterior to the southern 

 invasion into the existing area of Central America, two 

 streams of animals and plants passed between the great 



* Wallace, A. E., " Distribution of Animals,'' II., p. 45. 



s2 



