256 
ORIGIN OF LIFE IN AMERICA 
possess a remarkably discontinuous range. Rseudothclphusa 
colombiana inhabits the United States of Colombia. In the 
extreme western end of Panama, in the Chiriqui region, it 
occurs at a height of 4,000 feet, and a thousand miles north 
of this locality it is met with in Mexico. All along the great 
isthmus, on the other hand, there are isolated localities in 
which distinct species are found. Both the ranges of Unio 
and Pseudothelphusa in Central America seem to me more 
in agreement with the theory of the existence to the west of 
Central America of an ancient continuous land surface uniting 
Colombia and certain parts only of Central America, while the 
rest of the latter was still largely submerged. 
One of the chief zoogeographical features in the range of the 
land and fresh-water mollusks of Central America consists in 
the intermingling of South and North American forms within 
the limits of this area, rather than in the endemic species, and 
in the relationship of the molluscan fauna to that of the West 
Indies.* The affinity of the Antilles with South America, 
however, is much more pronounced than with North America, 
if we exclude Mexico. The distinctly endemic elements of 
Central America and the West Indian islands appear to have 
invaded both North and South America to some extent. 
The family Helicinidae for instance, as I mentioned (p. 157), 
is probably of semi-tropical origin, its range indicating that it 
has reached America from the west, establishing itself 
primarily in Central America and the West Indies at a very 
remote time of geological history. The operoulate land mol¬ 
lusks, to which the Helicinidae belong, are of great faunistic 
interest. Let us take for example the large family Cyclo- 
phoridae, which has its headquarters in southern Asia and 
southern Europe. In America the family is almost limited to 
Central and South America and the West Indian islands. 
Only in Mexico does it touch North American territory. As 
the geographical distribution of the family is now fairly well 
known, a great deal of interesting information can be derived 
from its study. The genus Cyrtotoma is peculiar to southern 
Mexico. Its nearest relations are Buckleyia of Colombia and 
Ecuador, and Crocidopoma of Haiti, Cuba and Jamaica. The 
* Martens, L. von, “ Mollusks of Central America,” p. xiv. 
