VARIATION DUE TO HEAT AND COLD 



85 



geisericola, 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 ]iredisposes 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 

 2Jomatia, H. nicicnsis, H. 'piscina, Leucocliroa 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 



Fig. 33. — Four examples of Lwi- 

 naea peregra Miill., from salt 

 marshes near the Sea of Aral, 

 showingdifferent effects prod need 

 by abnormal conditions of life. 



Fig. 34. — Four examples of Lini- 

 naea stagnalis L. , from marshes 

 in the Aral district wliich are salt 

 for several months in the year, 

 illustrating variation producedby 

 changes in the environment, x h. 



brown colour 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 production 

 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 2'rueJi us, Turho, Chiton, Mitra, and Pleurotoma, and is attested 

 by the specific names, not merely expressive of actual blackness 

 (e.g. nigerrimus, ater, atramentariiis, maurus), bvit also of a 

 generally lugubrious tone (e.g. moestus, funehrcdis, trisfis, higuhris, 

 luctuosus). It is highly probable that this concurrence of 

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



^ Journ. dc Concliyl. xxiii. 1875, p. 105. 



