Notes on South Africmi Mollusca. 



131 



was no barrier to prevent these large and vigorous snails from 

 spreading southward ; and southward they spread, exterminating 

 the Acavidae as they went. When, however, they reached the 

 south-west corner of Africa they found themselves on the horns 

 of a dilemma : while the winter in this region was too cold to be 

 agreeable to these snails of tropical origin, the summer was much 

 too dry. Consequently their distribution received a check, and even 

 to the present day very few members of the Achatininae have been 

 able to penetrate into the area which lies west of the twenty-second 

 degree of east longitude and south of the Tropic of Capricorn. In 

 this region, therefore, the Acavidae have been able to survive ; for 

 the Acavidae is such an old family that it has had time to become 

 acclimatized to all sorts of conditions, from the arid wastes of 

 Namaland to the forests of Brazil. 



Perhaps, in time, the Achatininae will evolve forms which are 

 able to flourish in this region also, and then the Acavidae will be 

 found no more on the x\frican continent. Bat it seems more likely 

 that the Helices which man has introduced into South Africa from 

 Europe will constitute the most formidable competitors of Trigo- 

 nephrus and Dorcasia. Civilization has upset the normal course of 

 the evolution and distribution of animals; and, if the world lasts 

 long enough, we might almost look forward to a time when all the 

 larger snails and slugs in the temperate regions of the Southern 

 Hemisphere will be of European origin. 



Diagram showing the possible spreading of the Acavidae. 



Approximate area supposed to have been submerged in early uiesozoic times. 

 Ditto ditto late mesozoic times. 



Ditto ditto earl}- tertiary times. 



If this history of the Acavidae is correct, the general course of the 

 evolution of the family might be summarized as follows. The Aca- 



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