256 ORIGIN OF LIFE IN AMERICA 



possess a remarkably discontinuous range. Pseudothelphusa 

 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 moUuscan 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 exteat. 

 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. 



