ii SEMPER'S EXPERIMENTS 81 



by a very simple experiment. Take two equal 

 volumes of water, 1,000 cubic centimetres for instance, 

 and put one of them into a broad and shallow basin, 

 so that it extends over a large surface, while the other 

 is poured into a spherical glass vessel, so that the 

 horizontal surface is very small. The two volumes 

 are equal, but their form is quite different. Into each 

 vessel, with an abundance of aquatic plants Myrio- 

 phyllum and Elodea especially : always submerged 

 sorts, so that they are not in need of a large surface, 

 and cannot interfere with it put one pond-snail 

 of the same brood, or cluster of eggs, recently 

 hatched. The difference after a few days is sur- 

 prising, and in the course of time it is seen that 

 the pond-snail of the large-surface vessel is much 

 larger than the other one. As the volume of water is 

 the same in both cases, we must conclude that in itself 

 the volume is not that which determines the variations 

 of growth, and also that Semper's interpretation 

 cannot be accepted, for, whether spherical or wide- 

 mouthed, the same quantity of the same water should 

 contain the same amount of Semper's hypothetical 

 matter. 



If, then, we cannot admit Semper's explanation, 

 what is the cause of the observed facts ? This 

 question may be answered by new experiments, in 

 which various conditions may be made to vary at 



G 



