84 EXPERIMENTAL EVOLUTION LECT. 



In a second series of experiments I varied the 

 volume of the water, but allowed the surface to 

 remain the same. For instance, into two vases of iden- 

 tical form and diameter I poured unequal volumes 

 of water. The surface was the same, but the volumes 

 were very different. In such cases, while the animals 

 were certainly larger in the larger volume, the differ- 

 ence was not considerable. The influence of volume- 

 variations is thus seen to be much less important than 

 that of surface-variations. 



These experiments seem to me to call for the 

 following interpretation. The volume of water is in 

 itself of comparatively small importance, especially 

 for some species of pond-snails, and the real influence 

 is exerted by surface. And surface operates in this 

 manner only : the larger it is, the more exercise the 

 animals are able to take. The L. auricularia, to which 

 the above-mentioned experiments refer more especially, 

 seems to dislike deep vessels, and moves usually 

 in the horizontal plane near the surface. If the 

 surface is small it moves but little, while if it is 

 large the animal moves a great deal. On the other 

 hand L. stagnalis seems generally to care less about 

 surface or depth, and to prefer living in the deep 

 parts of the vessels where it is always moving about. I 

 have seen it in one case live almost all the time in the 

 deepest part of its prison (a glass balloon with a long 



