338 OKIGIN OF LIFE IN AMEEICA 



book form we are better able to appreciate the advances in our 

 knowledge that are due to his investigations. His researches 

 were not confined to recent zoology. He has also dealt with 

 the subject from a palaeontological point of view, and has 

 even subjected the main features of the flora to a detailed 

 revision. All the same, he acknowledges that there are several 

 important problems connected with the geological history of 

 South America for which no satisfactory solution has as yet 

 been suggested. More than twenty years ago, Dr. von Ihering 

 announced that the close relationship of the Brazilian with 

 the African fresh -water mussels implied the former existence 

 of a land connection between South America and Africa. 

 But, whereas America as a whole is one of the richest regions 

 for fresh-water mussels in the world, Chile an'd Peru are 

 among the poorest. While east of the Andes there is a 

 luxuriant fauna of fresh-water mussels largely related to that 

 of Africa, we find only the genus Unio (in its wide sense) re- 

 presented on the western slope of the mountains. Dr. von 

 Ihering explains this and other similar facts by the supposi- 

 tion that in the east and the west there were originally similar 

 faunas, but while eastern South America obtained a rich stock 

 of immigrants across the land bridge from Africa, the great 

 mountain chain of the Andes, which was then commencing 

 to rise, prevented any further influx westward. Considering 

 that the fresh- water mussels (Unionidae) are well represented 

 even in Jurassic deposits, Dr. von Ihering* recognised that the 

 geographical distribution of these mollusks may be utilised 

 to advantage as indicators of very ancient, especially 

 Mesozoic, conditions of land and water. 



In the course of his studies the same author came to the 

 conclusion that from the Cretaceous to the end of the Pliocene 

 Periods, South America must have been completely separated 

 from North America. A real South American continent 

 existed only since Oligocene times. It then consisted of two 

 parts united by the narrow isthmus of the newly formed 

 Andes. He named these two old original constituents of 

 South America " Archiguiana " and " Archiplata." The 

 former comprised the highlands of Guiana and Venezuela, 



* Ihering, H. von, " Archhelenis and Archinotis," p. 79. 



