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 of 
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 cqast, 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. R., “ Distribution of Animals,” II., p. 45. 
s 2 
