FISHES OF CENTEAL AMEEICA 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 higher 

 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 



