380 ORIGIN OF LIFE IN AMERICA 



which are absent from Africa have arisen in South America 

 from the Characinidae and Siluridae since the separation of 

 the two continents. 



Dr. von Ihering's early recognition of the importance of 

 fresh -water faunas as an aid to the palaeogeographical studies 

 of the Palaeozoic and Mesozoic Eras, has led to very remark- 

 able results. Some of these I have indicated already. His 

 noteworthy discovery that some of the fresh-water bivalves 

 of South America have a "lasidium" larva, while all the 

 Unionidae possess a " glochidium," placed the family Mute- 

 lidae into quite a different position. The Mutelidae are a 

 family essentially South American and African, demonstrat- 

 ing clearly, as Dr. von Ihering * admits, the existence of a 

 land connection between South America and Africa during the 

 Mesozoic Era. 



The family Achatinidae includes some of the largest and 

 most conspicuous land-snails, Achatina achatina growing to a 

 length of seven inches. Their distribution, which is well 

 known, indicates an African centre of evolution. The early 

 members of the family are unlmown and should be looked for 

 in mid-Mesozoic deposits, according to Dr. Pilsbry.f The 

 same authority believes that some of the sections of the Acha- 

 tinidae migrated to South America before the interruption of 

 the land connection across the tropical Atlantic. Dr. Pilsbry, 

 who in his earlier volumes condemns the practice of throwing 

 hypothetical bridges across the oceans, now almost takes it as 

 a matter of course that Africa and South America were once 

 united by land. The mollusks, as most other groups of 

 animals, yield facts of distribution that are quite unexplain- 

 able by the theory of accidental dispersal, and many of those 

 who at first were firm believers in the immutability of our 

 ocean basins, have entirely changed their opinions after a 

 careful study of zoogeography . It is the smaller kinds of the 

 Achatinidae which show the affinity between the two con- 

 tinents most clearly. Thus the genus Subulina is confined to 

 Africa and tropical America while Opeas and Pseudopus have 

 spread to other parts as well. 



* Therm?, H. von, " Archhelenis and Archinotis," pp. 125 145. 

 t Pilsbry, H. A., " Manual of Conchology," Vol. XVIII., p. vi. 



