iv ENVIRONMENT AND DEFORMATION 207 



feed on one single sort of plant, acquire form and size 

 which differ from those of the same species fed on 

 other plants, and it even seems that here physiological 

 variation also comes into play, since prolonged life 

 on one plant makes them less dangerous for other 

 sorts of plants. 



Deformations may be determined by other natural 

 causes. M. Pire 1 has seen Planorbis complanatus much 

 deformed in Belgium through the influence of a thick 

 layer of aquatic plants on the surface of the pond, 

 preventing easy access to the air, and S. Clessin 2 ex- 

 plains the numerous deformations of the Lymnaea 

 tumida in the Bodensee and Starnbergsee as due to 

 the constant motion of the surface of these two lakes. 



Form may vary in plants also, according to motion 

 for instance, many plants being much smaller and 

 much deformed in windy localities. Behrens has 

 noticed the influence of currents on the forms of 

 aquatic plants, which E. Mer and others have also 

 done ; and this leads us to consider the variations in 

 internal structure, and in functions which are cor- 

 related with those modifications of external characters. 

 E. Mer is among those who have investigated the 

 matter in the most precise manner, and the results 



1 Annales Soc. Malacologiqne dc Belgiqtie, vol. vi., 1871, and xiv., 

 1879. 

 a Deutsche- Excursions Molhisken Fauna, Nuremberg, 1876, p. 368. 



