132 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



forms, therefore, enter into the category of local 

 races. 



This scattered polymorphism is still more accentu- 

 ated in other species. In the Helix striata, to be 

 found nearly throughout France, save on the high 

 mountains, there can easily be distinguished twenty 

 modes of variation which, grouped together, may 

 give a total of 1458 combinations. The attribution 

 of a specific name to each of these shades would be, 

 therefore, irrational, and would only impede the 

 expose of general facts which the consideration of 

 modes allows us to set forth. Thus, in flinty 

 regions the shells are thin, mode tennis ; in the 

 limestone regions they are thick, mode solidus. 

 In the Paris Basin, the northern part of the domain 

 of the species, the shells are large and the test is 

 thin ; in the warm and dry regions of the south, as 

 in Provence, the shells are small with a thick test, 

 and the inner membrane of the peristome becomes 

 larger and of a pink colour ; lastly the epidermis 

 is adorned with dark-coloured bands, sometimes 

 almost touching each other. These two modes 

 might receive the names of Septentrionalis and 

 Meridionalis. When the influence of the environ- 

 ment has the effect of reducing the period of 

 development, whether this is occasioned by heat 

 or cold, the spiral has half a turn or a whole turn or 

 several turns less than usual ; and at the same time, 

 some of the characters of the adult, the structure of 

 the peristome, or the deviation of the spiral, appear 

 prematurely, mode prematurus, in opposition to a 

 mode productus observed more rarely in subjects 

 endowed with greater vitality or placed in excep- 



