Ill 



VARIATION DUE TO HEAT AND COLD 



85 



from the hot water of the Iceland geysers, are alike thin and 

 dwarfed forms. 



Many instances may be given of ' varieties due to locality.' 

 In some of these, the cause which predisposes towards variation 

 can be inferred with some approach to certainty, in others we 

 must be content to note the fact, without at present being able 

 to perceive its explanation. 



Desert specimens of widely distributed species, e.g. Helix 

 pomatia^ IT. niciensis^ IT. pisana^ LeucocJiroa candidissima are 

 much thicker than the type, and tend to lose all trace of coloured 

 bands. These modifications are clearly the means of preventing 

 evaporation of moisture, the dull white or grayish brown colour 



Fig. 33. — Four examples of Lim- 

 naeaperegra MiilL, from salt 

 marshes near the Sea of Aral, 

 showing different effects pro- 

 duced by abnormal conditions 

 of life. 



Fig. 34, — Four examples of Lim- 

 naea stagnalis L., from marshes 

 in the Aral district which are 

 salt for several months in the 

 year, illustrating variation pro- 

 duced by changes in the envi- 

 ronment. X h. 



being calculated to absorb the smallest possible amount of heat. 

 Desert shells in all parts of the world (^e.g. N. Africa, Arabia, 

 Central Asia, S. Africa, W. America) have been noticed to 

 exhibit these peculiarities. 



A very singular case of the reverse process, i.e. the produc- 

 tion of darkened forms of shell through cold, has been noticed 

 by Fischer as characteristic of the marine shells of the west coast 

 of South America.^ This melanism is especially noticeable in 

 Trochus, Turbo, Chiton, Mitra, and Pleurotoma, and is attested 

 by the specific names, not merely expressive of actual blackness 

 (e.g. nigerrimus, ater, atramentarius, maurus), but also of a gen- 

 erally lugubrious tone (e.g. moestus, funebralis, tristis, lugubris, 

 luctuosus). It is highly probable that this concurrence of 

 specific melanism (which stands quite alone in the world) is 

 1 Journ. de Conchyl. xxiii. 1875, p. 105. 



