128 REPORT — 1839. 



increase of numbers in the central districts, as compared with 

 some of the southern, indicates an influence in some cases 

 more powerful than climate. This influence we shall find to 

 be in its nature geological. It is the influence of the struc- 

 ture of a country on its existing Fauna. 



The influence of the various kinds of rocks is very diflTerent 

 and very important. Certain species, and even certain genera, 

 appear to prefer certain rocks ; but of all rocks limestone 

 is the most favourable to the number and propagation of spe- 

 cies. All kinds of lime rocks are not equally indifferent. Cer- 

 tain mollusks appear to prefer chalk, others oolite, others moun- 

 tain limestone. Thus we find Helix carthusianella chiefly asso- 

 ciated with chalk. Helix pomatia and carthusiana with the older 

 tertiary, the cretaceous and oolitic formations, and Helix scar- 

 burgensis with the coal formation. Other species are distributed 

 over all varieties of limestone rocks, but are not found on other 

 kinds of soil, such as Helix glabella, Helix lapicida, and Pupa 

 secale. The influence of all other rocks appears to be rather 

 negative than positive ; for, though several species limited in 

 distribution seem confined to certain rocks in our country, we 

 find them in other countries, where their distribution is more 

 general, indifferent as to the soil in which they live. In ge- 

 neral it is the mineralogical character of the rock which in- 

 fluences rather than its age. Limestone and sand influence all 

 species as regards propagation, individuals multiplying to a 

 much greater extent on calcareous and sandy soils than on slates, 

 clay, or granite. Basalt has a similar influence, and primitive 

 rocks generally are unfavourable either to the development of 

 species or individuals. In certain cases the influence even 

 of limestone may be completely neutralized by climate, as we 

 find in Shetland, where the limestone tracts present no ex- 

 ceptions to the general paucity of species and individuals in 

 those islands. Climate may also be seen overpowering the 

 negative geological influence in Guernsey, where Helix varia- 

 bilis multiplies to a great extent along M^ith several other spe- 

 cies on the unfavourable surfaces of granite and quartz rock. 

 In some localities certain species are confined to certain rocks, 

 which are generally distributed over all soils in others. 



The order of influence of rocks on species is as follows, com- 

 mencing with the most influential : 



1. Cretaceous and oolitic. 



2. Carboniferous rocks and trap. 



3. Tertiary. 



4. Saliferous. 



5. Slates. 



6. Granite and gneiss. 



