TAYLOR : ON THE VARIATION OF MOLLUSCA. 30I 



colored and strikingly marked variations which have received 

 numerous distinctive names from Italian Limacologists. 



Limax arbonim gives similar testimony as in the northern 

 and north-western parts of this country and on the summits of 

 the mountains in Italy it becomes very dark in color, and loses 

 that glaucous appearance which distinguishes it on warmer or 

 drier grounds. On the Italian mountains the transition from 

 the ordinary to the intensely dark form can be traced. 



Cold is therefore a powerful factor in inducing the forma- 

 tion of black pigment, but it is extremely probable according to 

 the observations of some of our most able conchologists, that 

 humidity has also very great subsidiary influence in inducing 

 this variation. 



Lord Walsingham in his address to the Yorkshire Natural- 

 ists' Union on Melanism in Lepidoptera has pointed out that 

 the black color in insects may be a favorable factor in cold 

 regions, from its power of absorbing such few heat rays as there 

 may happen to be, and this may possibly be some explanation 

 of the black color of these Alpine varieties. 



Heat if too great in amount induces torpor in land and 

 freshwater species. Limncea stagnalis is said to be unable to 

 endure a temperature of more than 90°. The effect on the 

 secretion of shell of too much warmth, is similar to that of too 

 much cold, the result being a shell of a frail and delicate char- 

 acter, thus L. peregra var. thervialis from the warm springs 

 of the Pyrenees and the Vosges, and the var. steenstrupi from 

 the warm water of the Icelandic Geysers are equally remarkable 

 for their thinness and dwarfed character. 



Helix Pisana in warm sunny places becomes tinged of 

 beautiful rose color, at the mouth of its shell, which color is said 

 to be deficient in those specimens living in less favoured spots. I 

 have already pointed out under arid or desert conditions, the 

 effects of heat when accompanied by dryness — when however 

 associated with moisture and rich vegetation it produces some 

 of the most richly colored and finest shells which are known 

 to conchologists. 



