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. MoUusca living at only a shallow depth (e.g. 

 Tellina hcdtJdca, Mya arenaria, CarcUum edule) do not differ 

 much in size in diflerent 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 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 

 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 temperatm-e 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 'pere.gr a 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. 



^ Mobius, Report on ' Pommcrania' Exped. pp. 138-141. 



