CHAP, xxiii.] SUMMARY AND CONCLUSION. . 551 



organisms have spread over the earth, but owing to their small 

 size and rapid multiplication, they have made use of some which 

 are exclusively their own. Such are the passage along moun- 

 tain ranges from the Arctic to the Antarctic regions, and the 

 dispersal of certain types over all temperate lands. It will 

 perhaps be found that insects have spread over the land surface 

 in directions dependent on our surface zones — forests, pastures, 

 and deserts ; — and a study of these, with a due consideration of 

 the fact that narrow seas are scarcely a barrier to most of the 

 groups, may assist us to understand many of the details of 

 insect-distribution. 



Terrestrial Mollusca. 



The distribution of land-shells agrees, in some features, with 

 that of insects, while in others the two are strongly contrasted. 

 In both we see the effects of great antiquity, with some special 

 means of dispersal ; but while in insects the general powers of 

 motion, both voluntary and involuntary, are at a maximum, in 

 land-molluscs they are almost at a minimum. Although to 

 some extent dependent on vegetation and climate, the latter are 

 more dependent on inorganic conditions, and also to a large 

 extent on the general organic environment. The result of these 

 various causes, acting through countless ages, has been to spread 

 the main types of structure with considerable uniformity over 

 the globe ; while generic and sub-generic forms are often 

 wonderfully localized. 



Land-shells, even more than insects, seem, at first sight, to 

 require regions of their own ; but we have already pointed out 

 the disadvantages of such a method of study. It will be far 

 more instructive to refer them to those regions and sub-regions 

 which are found *to accord best with the distribution of the 

 higher animals, and to consider the various anomalies they pre- 

 sent as so many problems, to be solved by a careful study of 

 their habits and economy, and especially by a search after the 

 hidden causes which have enabled them to spread so widely 

 over land and ocean. 



The lines of migration which land-shells have followed, can 



YoL. II.— 36 



