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. Ilis 
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 unknown and should be looked for 
in mid-Mesozoic deposits, accoixling to Dr. Pilsbry.f The 
same authority believes that some of the sections oT the Aoha- 
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. 
* Iherincr, H. von, “ Archhelenis and Archinotis,” pp. 125 — 145. 
t Pilsbry, H. A., “ Manual of Conchology,” Yol. XVIII., p. vi. 
