76 ZOOLOGICAL GEOGEAPHY. [PART in. 



are, they will be more effective where the islands have been long 

 separated from the mainland, as is here undoubtedly the case. 

 It seems most probable that the great development of land- 

 shells in islands, is due to the absence or deficiency of the verte- 

 brata, which on continents supply a variety of species adapted 

 to prey upon these molluscs. This view is supported by the fact, 

 that in such islands as have been united to a continent at no 

 very distant epoch, and still maintain a continental variety of 

 vertebrata, no such special development of land-shells has taken 

 place. If we compare the Philippine islands with the Sunda 

 group, we find the development of vertebrata and land-molluscs 

 in inverse ratio to each other. The same thing occurs if we 

 compare New Zealand and Tasmania ; and we have a still more 

 striking example in the Antillean group itself, continental 

 Trinidad having only 20 genera and 38 species, while the 

 highly insular Jamaica has about 30 genera and more than 500 

 species. 



The other causes favourable to the increase and development 

 of land- shells are of a physical nature. A great extent of lime- 

 stone-rock is one ; and in the larger West Indian islands we have 

 a considerable proportion of the surface consisting of this rock. 

 But perhaps equally or more important, is the character of the 

 land surface, and the texture of the exposed rock itself. A 

 much broken surface, with numerous deep ravines, cutting up 

 the whole country into isolated valleys and ridges, seems' very 

 favourable to the specialization of forms in this very sedentary 

 class of animals. Equally favourable is a honeycombed and 

 highly-fissured rock-surface, affording everywhere cracks and 

 crannies for concealment. Now, taking Jamaica as an example 

 of the archipelago, we find all these conditions in a wonderful 

 degree. Over a large part of this island, a yard of level ground 

 can hardly be found ; but ridges, precipices, ravines, and rock- 

 bound valleys, succeed each other over the whole country. At 

 least five-sixths of the entire surface is limestone, and under the 

 influence of tropical rains this rock is worn, fissured, and honey- 

 combed, so as to afford ample shelter and concealment for land- 

 shells. 



