(a) Changes in Climate, Temperature, Elevation, etc.—In the eastern basin of the Baltic the marine Mollusca are much more stunted than in the western.[185] For instance, Mytilus edulis near Kiel is 8–9 cm. long, while near Gothland it only attains a length of 3–4 cm. Mollusca living at only a shallow depth (e.g. Tellina balthica, Mya arenaria, Cardium edule) do not differ much in size in different parts of the Baltic, but in the far eastern basin the calcareous layers of the shells of Mya arenaria and Tellina balthica are extraordinarily thin, and disappear very rapidly after death, leaving only the cuticular membrane, still united by the ligament, in a perfect state of preservation. These remarkable variations are no doubt to a large extent due to the violent changes of temperature which are experienced in the Baltic, and by which the steady development of the animals in question is interrupted and thrown out of gear. The same species occur on the coasts of Greenland and Iceland, where they attain a considerably larger size than in the Baltic, in spite of the lower mean temperature, probably because their development is not interrupted by any sudden change from cold to heat or vice versâ.
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.
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.
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.
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.[186] 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 generally 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 due to the cold polar current which impinges on the Chilian coasts, for the same genera occur on the opposite shores of the continent without exhibiting any trace whatever of this mournful characteristic.
It is a well-known fact, attested by many observers, that our common Limax agrestis as well as the young of Arion ater become decidedly darker in summer than in winter. If these slugs were accustomed to disport themselves in the sun, it might have been suggested that this increased darkness of colour tended to absorb more of the heat rays. But since this is not the case, the result is probably due to some unexplained effect of higher temperature. According to Lessona and Pollonera, the length of the keel in Limax arborum varies greatly in different parts of Italy, being shorter in specimens from low ground, but much longer in those inhabiting more elevated regions. The longer the keel, the more obscure the colouring becomes, so that in the Upper Alps of Piedmont individuals are practically black. Roebuck has observed that Scottish specimens of this same slug are much darker and less translucent than English forms. According to Simroth, our common black slug, Arion ater, is a northern type, which in more southern latitudes assumes the form known as A. rufus. Similarly Limax maximus “in its northern form cinereo-niger is almost wholly black, but in the more genial climate of Italy develops a series of brilliantly coloured and strikingly marked variations which have received numerous distinctive names from Italian limacologists.”[187] According to Scharff, however[188] (who regards the colours of slugs as in the main protective), these dark forms are by no means exclusively northern, being found equally on the parched plains of Spain and Portugal, and in the bleak climate of Norway. The same authority observes that similar forms occur both in the dry regions of E. Germany, and in the very humid district of western Ireland.