84 EFFECTS OF CHANGES IN THE ENVIRONMENT chap. 



stunted than in the western. ^ 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. TelUna halthica^ My a 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 halthica 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 spe- 

 cies 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 versd. 



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 pro- 

 duction 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 gen- 

 erally of a very delicate form and dwarfed habit, while the small 

 variety known as lacustrls 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, whicli 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 tlie Pyrenees and the Vosges, and the var. geisericola, 

 ^ Mobius, Bcport on ' Pommerama'' JExped. pp. 138-141. 



