64 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



standing in the fixity of specific forms. But there 

 exists in the present world a certain number 

 of groups which, on the contrary, show a tendency 

 to variation without end. Such are, for example, 

 among continental molluscs, the Melanopses of the 

 Mediterranean basin ; their forms are connected 

 with one another by so many transitions that the 

 distinction of species lacks any exact basis. Some 

 scholars have divided these shells into three genera 

 and a number of species, while others unite all these 

 forms into one or two species at the most. 



Such, again, are the Helices of the Iberus group, 

 widely spread in Sicily. Their variations are con- 

 siderable, and the extreme types represented by 

 a tall and globular form and by a very squat and 

 carinated [keel-shaped] form are very dissimilar. 

 Each variety is special to a locality limited in extent ; 

 but in two neighbouring districts there can be 

 observed transitional forms quartered in the inter- 

 mediate zone. Thus, taken as a whole, this Sicilian 

 group forms a continuous series. If, from one cause 

 or another, certain intermediate zones were to 

 be depopulated, the series would become divided 

 into very distinct groups, the connection of which 

 with each other would become impossible to demon- 

 strate. 



The Achatinellce of the Sandwich Islands have 

 recently made known to us a phenomenon of 

 dissociation of this kind. The group is special to 

 the Sandwich Archipelago, where more than two 

 hundred species have been observed. The large 

 Island of Hawaii raises but six species, while the 

 Island of Oahu, which is one sixth of the size, con- 



