CHAP. T DISPERSAL OF ANIMALS AND PLANTS 79 



their powers of voluntary dispersal, even on land, are very 

 limited, and this will explain the extreme restriction of their 

 range m many cases. 



Great Antiquity of Land-Shells. — The clue to the almost 

 imiversal distribution of the several families and of many 

 genera, is to be found, however, in their immense antiquity. 

 In the Pliocene and Miocene formations most of the land- 

 shells are either identical with living species or closety 

 allied to them, while even in the Eocene almost all are of 

 living" genera, and one British Eocene fossil still lives in 

 Texas. Strange to say, no true land-shells have been 

 discovered in the Secondary formations, but they must 

 certainly have abounded, for in the far more ancient 

 Palaeozoic coal measures of Nova Scotia two species 

 belonging to the living genera Pupa and Zonites have been 

 found in considerable abundance. 



Land-shells have therefore survived all the revolutions 

 the earth has undergone since Palaeozoic times. They 

 have been able to spread slowly but surely into every land 

 that has ever been connected with a continent, while the 

 rare chances of transfer across the ocean, to which we have 

 referred as possible, have again and again occurred during 

 the almost unimaginable ages of their existence. The 

 remotest and most solitary of the islands of the mid-ocean 

 have thus become stocked Avith them, though the variety 

 of species and genera bears a direct relation to the facilities 

 of transfer, and the shell fauna is never very rich and 

 varied, except in countries which have at one time or other 

 been united to some continental land. 



Causes Favouring the Abundance of Land-Shells. — The 

 abundance and variety of land-shells is also, more than that 

 of any other class of animals, dependent on the nature of 

 the surface and the absence of enemies, and where these 

 conditions are favourable their forms are wonderfully 

 luxuriant. The first condition is the presence of lime in 

 the soil, and a broken surface of country with much rugged 



water in calm weatlier, the distance might extend to a thousand miles or 

 more. The eggs of fresh-water mollusca, as well as the young animals, are 

 known to attach themselves to the feet of aquatic birds, and this is probably 

 the most efficient cause of their very wide diffusion. 



