FISHES OF CENTRAL AMERICA 
235 
in North America occurred in post-Tertiary times, for he 
believes Central America to have been still submerged during 
the Pliocene Period. The term “ post-Tertiary,” I may men¬ 
tion, is synonymous with Quaternary or post-Pliocene. Any 
geological deposits more recent than Pliocene come within 
the meaning of that term. But the skeletons of the great 
South American Mylodon and Megalonyx certainly occur in 
Texas in true Pliocene beds. I do not think there is any geo¬ 
logist in America now who would uphold the Pleistocene, or 
even less the post-Glacial age of these deposits. My conten¬ 
tion, therefore, is that the northward advance from South 
America is a pre-Glacial or pre-Pleistocene event. 
Now one of the most remarkable and astonishing features 
of that faunistically so peculiar continent of South America 
is that, whereas its tropical fauna has very little affinity 
with the fauna of North America, the more remotely placed 
Chilean and Patagonian faunas present in some groups of 
animals a striking resemblance to it. This character will be 
more fully dealt with in another chapter (pp. 410—419). It 
may be mentioned, however, that numerous groups, and even 
species, of northern plants are met with in Chile, which are 
wholly, or almost entirely, absent in the intervening region, 
occupying an area of thousands of miles. Northern genera of 
butterflies and beetles, such as Argynnis, Colias and Carabus, 
all of which are almost unknown in the countries immediately 
south of Mexico, reappear in numbers in the extreme southern 
tip of South America. Dr. Wallace thought that this south¬ 
ward migration of northern forms of animal life must have 
been effected mainly 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, while storms might often have 
carried insects from peak to peak, over intervening forest 
lowlands, or narrow straits of sea. Dr. Wallace’s idea that the 
mountains all along Central America were formerly higlier 
than they are now and sustained northern forms of animal 
life is not supported by any evidence. Considering that he 
imagined the long isthmus to have been slowly rising from the 
sea since pre-Glacial times, Dr. Wallace’s suggestion that 
the mountains were so much higher during the Glacial Epoch 
