CHAP. XIV.] THE NEOTROPICAL REGION. 83 



of interchange of the mammals or land-birds of the two 

 countries, no less than by the very fragmentary nature of the 

 resemblances that do exist. The northern element consists 

 almost wholly of insects ; and is evidently due to the migration 

 of arctic and north temperate forms along the ridges and 

 plateaus of the Andes ; and most likely occurred when these 

 organisms were driven southward at successive cold or Glacial 

 periods. 



A curious parallel exists between the past history and actual 

 zoological condition of South America and Africa. In both 

 we see a very ancient land-area extending into the South 

 Temperate zone, isolated at a very early period, and developing 

 only a low grade of Mammalian life ; chiefly Edentates and 

 Rodents on the one. Lemurs and Insectivora in the other. Later 

 we find an irruption into both of higher forms, including 

 Quadrumana, which soon acquired a large and special develop- 

 ment in the tropical portions of each country. Still later we 

 have an irruption into both of northern forms, which spread 

 widely over the two regions, and having become extinct in the 

 land from whence they came, have been long held to be the 

 original denizens of their adopted country. Such are the 

 various forms of antelopes, the giraffe, the elephant, rhinoceros, 

 and lion in Africa ; while in America we have deer and peccaries, 

 the tapir, opossums, and the puma. 



On the whole, we cannot but consider that the broad outlines 

 of the zoological history of the Neotropical region can be traced 

 with some degree of certainty ; but, owing to the absence of 

 information as to the most important of the geological periods 

 — the Miocene and Eocene — we have no clue to the character of 

 its early faima, or to the land connections with other countries, 

 which may possibly have occurred in early Tertiary times. 



