Karl Semper has shown that _Limnaea stagnalis_ is developed, lives and feeds best in a mean temperature of about 20° C. (= 68° F.). This mean, however, must not be the mean of two distant extremes, for the _Limnaea_ cannot digest its food and grow in a temperature which is less than 14° or 15° C. (= 57° or 59° F.), or more than 30° to 32° C. (= 86° to 90° F.). In certain localities, therefore, the interruption to the growth of this species must be serious and prolonged, and may tend towards the production of more or less dwarfed varieties. Thus specimens from Malham Tarn, a lake in Yorkshire 1250 feet above the sea, are permanently dwarfed, and have a very thin and fragile shell. _Limnaea peregra_ in the Pyrenees, Alps, and Himalayas is generally of a very delicate form and dwarfed habit, while the small variety known as _lacustris_ occurs, according to Jeffreys, only in mountain lakes in Zetland, Scotland, Ireland, and N. England. Specimens brought by Mr. Bateson from lakes near the Sea of Aral, which are salt for some months and comparatively fresh for others, exhibit clearly the effect of changes in the environment (Figs. 33 and 34). Excess of heat produces similar results to excess of cold. _L. peregra_ var. _thermalis_, found in the warm springs of the Pyrenees and the Vosges, and the var. _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 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. [Illustration: FIG. 33.--Four examples of _Limnaea peregra_ Müll., from salt marshes near the Sea of Aral, showing different effects produced by abnormal conditions of life.] [Illustration: FIG. 34.--Four examples of _Limnaea stagnalis_ L., from marshes in the Aral district which are salt for several months in the year, illustrating variation produced by changes in the environment. × ½.] Desert specimens of widely distributed species, _e.g._ _Helix pomatia_, _H. niciensis_, _H. pisana_, _Leucochroa 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 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.