Quarterly Journal of Conchology. 177 



cause is removed, e. g., the var. tenuis of Helix aspersa found in 

 places poorly provided with carbonate of lime — like the Channel 

 Islands. I do not consider that this variation would ever develope 

 into a distinct species. 



We now have reached the lowest degree — that of individual va- 

 riation. This varies considerably in different species. In Helix 

 picta it is great, one may have some dozens of specimens and not 

 two exactly alike in colour and markings. In some of the Neritinx 

 there is even greater diversity; in the largest suite of specimens it 

 would be difficult to find any two between which some slight differ- 

 ence would not be detected. Other species on the contrary are 

 remarkable for the almost perfect similarity of all the specimens. 

 I once had over 50 specimens of the Porto Rican Clausilia tridens, 

 and to use a common expression they were all as "alike as two 

 peas". This was accounted for in my opinion by the West Indies 

 being an unfavourable climate for Clausilia, and the same causes 

 that have prevented the genus extending itself then have kept the 

 single species true to its original form. 



When individual variations are also malformations due to patho- 

 logical causes they are termed "monstrosities". 



In the practical application of these rules to particular cases, 

 there will often be a diversity of opinion, especially as it is not always 

 possible to test hybridity by actual experiment. Especially with 

 regard to the marine moUusca there would be almost insuperable 

 difficulties to doing this in most cases, and it is for this reason that 

 in my opinion a practical, though empirical test like that laid down 

 by Dr. Kobelt becomes valuable. 



Mr. Simpson's illustrations of Clausilia biplicata and laminata 

 was certainly — as pointed out by Mr. Marshall — a most unfortunate 

 one. Not only are these species most readily distinguishable, but 

 they belong to quite different subgenera. 



That characters have varying importance according to circum- 

 stances is a well known and recognized fact, and must be the case 

 in a natural classification, which is founded not on particular organs 

 arbitrarily taken as the basis of arrangement, as the pistils and 

 stamens in the Linnean botanical arrangement, but on the general 



