288 THE VARIATION OF ANIMALS IN NATURE 



related to environmental factors, we think it better for the 

 present to regard it as an open question whether, as Regan 

 suggests, it has an adaptive significance. 



9. Functional significance of ribbing of shell in Helicigona 

 cingulata {Boettger, IQ32, p. 2og) . 



Boettger observed that there is a high frequency of snails 

 with strongly ribbed shells in the Alps. He devised an ap- 

 paratus by which the shells could be subjected to crushing by a 

 measurable force, and used it for testing the resistance to crush- 

 ing shown by the smooth Helicigona cingulata colubrina and the 

 ribbed H. c. gobanzi. Ten of each species were used, and 

 Boettger found that colubrina was crushed at an average weight 

 of 1,420 grammes, and gobanzi at an average of 1,506. From 

 this he concluded that the ribs are adaptive, as they serve 

 to strengthen the shell against crushing, and the high 

 frequency of ribbed forms in the Alps is due to their greater 

 power of resisting falling stones. 



It will be noted that the difference between the two varieties 

 in the matter of their resistance is not very great ; but it is 

 perhaps enough to give the ribbed form sufficient selective 

 advantage. Boettger's case is not very well made out. He 

 says nothing specific about the distribution of ribbed and un- 

 ribbed forms and their frequency in places where stone-falls 

 are likely to be of regular occurrence. He simply affirms that 

 ribbed forms are more common in the Alps. He certainly 

 points out that ribbed forms of Arianta arbustorum occur in the 

 Alps and are never found in the plains. He disposes of the 

 suggestion which has been already made, that the ribbing is a 

 ' Kaltanpassung,' by pointing out that ribbing does not become 

 more frequent towards high latitudes. 



As to the two varieties in question, not only does Boettger 

 not give any figures for their frequency of occurrence in the 

 relevant habitats, or any statement as to whether the ribbed 

 form is more dominant in places exposed to avalanches and 

 rock-falls, but he does not even say where his specimens came 

 from. It seems that gobanzi is restricted to the upper Val 

 Sarca, near Candino (Val Vestino), and has a very limited 

 range there (Kobelt, 1876, p. 37: 'auf eine Kleine Strecke 

 beschrankt, aber dort in Menge . . .') in a kind of enclave 

 in the colubrina area, where indeed (Kobelt, I.e.) they seem 

 to live in contact. Kobelt points out the highly interesting 



