260 
ORIGIN 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. 
