TAYLOR : ON THE VARIATION OF MOLLUSCA. 295 



equivalent in value to our varietal ones, tends to divorce the 

 form so named from its immediate and close allies, as is so 

 clearly shown by the Rev. Professor Bonney in his address to 

 the Mineralogical Society, where he forcibly points out the ad- 

 vantages of the employment of modifying terms instead of dis- 

 tinct and separate names to express the minor modifications or 

 differences in the constituents of minerals, and which remarks 

 are singularly applicable to our own study, he says, the one 

 method — giving distinct or what we should call specific names 

 — accentuates the distinction and loses sight of the relation, the 

 other — that of applying modifying, or what may be considered 

 equivalent to our varietal names — whilst noting the distinction, 

 keeps prominent the relationship. 



This last point is to some extent brought about by our 

 gifted continental friends by the system of grouping which they 

 adopt, which groups are I believe nearly if not quite co-ordinate 

 with the old Linnean and Lamarckian species, thus the group 

 Stagnaliana would in England be considered synonymous with 

 the Linnean species Stag?ialis and the different forms composing 

 this group would be to our English views, varieties of that 

 species. 



I will now very briefly consider some of the causes of 

 modification in our land and freshwater shells, and as far as I 

 am able, show the direction in which the various forces operate. 

 I shall to make the illustrations clearer and more definite, borrow 

 some of the instances from foreign countries, so that we can 

 note by comparison the same change in a modified degree, 

 amongst our own species. 



Environment is the most powerful and perhaps the only 

 force inducing variation, or rather fostering those variations 

 which are most in accordance with the surrounding conditions, 

 and I shall give one or two instances where the influence of the 

 surroundings have evolved variations in harmony with the forces 

 exercised. We are, however, often unable to recognize the 

 cause of many modifications, but as it is patent that no change 



