CH. Ill] GEOLOGICAL STRUCTURE AND DISTRIBUTION. 137 



the geological structure of the locality has a great in- 

 fluence in the matter. Prof. Edward Forbes held that the 

 snails were decidedly influenced by such causes in their 

 range. Limestone is as might be supposed the most 

 favourable rock for their full development ; but according 

 to him a sandy soil is better than a clayey or slaty 

 substratum. Certain apparent exceptions to this general 

 statement are explained by the greater influence of 

 climate ; thus the Shetland Islands abound in limestone 

 and yet there is a paucity of shells; 011 the other hand the 

 island of Guernsey is populated by vast numbers of 

 specimens of a particular species of snail ; this is not due 

 to the unfavourable granite soil but rather to the favour- 

 able climate, which is a stronger influence than the soil. 

 Dr Gadow has recently put forward again the importance 

 of the geological terrain in affecting the distribution of 

 organisms. It is much more important thinks Dr Gadow 

 than temperature or altitude. His studies upon the 

 subject were made in Portugal, and communicated to the 

 meeting of the British Association at Bath in the year 

 1888. They relate principally to reptiles and amphi- 

 bians ; Dr Gadow found reasons for coming to the con- 

 clusion that for both these groups of animals the red 

 sandstone was by far the most favourable soil. There is of 

 course in these two cases no pretence that the soil has a 

 direct effect comparable to the limy soil which is stated 

 to be so advantageous to land Mollusca. 



