260 OKIGIN OF LIFE IN AMERICA 



continents, leaving certain traces of their transit in the 

 more ancient portions of the country. And yet I believe, and 

 shall produce ample evidence in support of my contention, 

 that only certain fragments of Central America formed part 

 of that land which long ago served as the highway between 

 North and South America. This fact is not so readily re- 

 vealed from a study of the Central American animals and 

 plants. All we can gather from our present researches is 

 that there are certain ancient elements in the fauna and flora 

 of Central America exhibiting affinities with North America, 

 Asia, Europe, Africa, the West Indies and South America, and 

 that these older elements are being dispossessed or driven into 

 the more inaccessible parts by the members of the new and 

 most recent invasion from the south which traversed the newly 

 formed Central American isthmus. It is believed that this 

 must have commenced in Pliocene times. Of the two marine 

 barriers which previously prevented this southern advance, 

 one was in the neighbourhood of the Panama Canal, the other 

 at the Isthmus of Tehuantepec, but to judge from the animals 

 and plants of Central America,, the former had already dis- 

 appeared when the more northerly one was still in existence. 



