REPORT ON INVESTIGATION OF LAND MOLLUSCA 281 



explained by their entry into these islands having been 

 originally effected on the eastern coast, from whence their 

 progress northward is more readily effected ; inasmuch as 

 the western coast can only be reached by crossing the 

 intervening country and overcoming the many physical 

 obstacles to their progress, though the relative inferiority 

 of the life of the western regions facilitates the advance 

 of the invaders from the east when once the western 

 slope has been attained. This is further demonstrated by the 

 material at our command, which shows that the molluscs 

 penetrate farther north along the eastern than the western 

 side of Scotland. 



Helix aspersa, for instance, is one of the most recently 

 evolved and dominant species of Mollusca, its superiority 

 being displayed not only by its successful competition with 

 all other species for possession of favourable areas, but by 

 its remarkable power of colonisation, exercised to the 

 detriment and destruction of the aboriginal species of those 

 portions of the globe in which life is on a lower plane; and 

 its superiority is further shown by the greater perfection 

 of its organisation, and the more decided concentration of 

 the various nervous elements which constitute in these 

 creatures what represents the brain in the higher animals. 

 This species is recorded on the east side as far north as 

 Elgin and Banff, and on the west on the Isles of Iona, Eigg, 

 and Lismore, and at the Castle of Ardtornish. 



Helix nemoralis on the east reaches only to St Cyrus, 

 and the Isle of Lismore on the west. It also is a dominant 

 and advancing species of a high type of organisation, with 

 much greater biochemical power of obtaining sustenance 

 from a limited amount of food than less advanced species. 

 Its place further north is taken by its close ally and phylo- 

 genetic predecessor, H hortensis, which is a form retreating 

 slowly before its successor, H. nemoralis. 



Hygromia rufescens, H hispida, and Ena obscura have a 

 similar range of distribution to that of H. nemoralis, though 

 further research northward is necessary to enable us to 

 establish their precise limits. 



All these examples are of large and conspicuous forms, 



24 2 N 



