CHAP, v.] 



DISPERSAL OF ANIMALS AND PLANTS. 



77 



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 Palseozoic 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 Palseozoic 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 with 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 abun- 

 dance and variety of Jand-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 rock offering crevices for 

 concealment and hybernation. The second is a limited bird 

 and mammalian fauna, in which such species as are especially 

 shell-eaters shall be rare or absent. Both these conditions are 

 found in certain large islands, and pre-eminently in the Antilles, 

 which possess more species of land-shells than any single con- 

 tinent. If we take the whole globe, more species of land-shells 

 are found on the islands than on the continents — a state of 

 things to which no approach is made in any other group of 

 animals whatever, but which is perhaps explained by the 

 considerations now suggested. 



The Dispersal of Plants. — The ways in which plants are dis- 

 persed over the earth, and the special facilities they often possess 



