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 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 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 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 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 
