86 EXPERIMENTAL EVOLUTION LECT. 



with the water around them, through the muslin, whose 

 only function was to prevent the animals from 

 escaping from the tubes into the vessel, or vice versa. 

 In all such experiments, the water being the same in 

 both tubes and vessel, food being superabundant, 

 temperature identical, and surface and volume only 

 being different, I have seen the pond-snails in the 

 tubes remain much smaller than those in the vessel. I 

 may even add that in some cases I have had one tube 

 as above described, and another stopped at the lower 

 end with a good cork and wax around it, so that the 

 water in the tube never got mixed with that of the 

 vessel, and have hardly if at all noticed any difference 

 in the dimensions of the animals of both tubes. 



This experiment may be performed in another 

 manner by fixing up a sort of cage (with muslin and 

 glass rods), which affords more space than the tubes, 

 and more surface and volume ; the communication 

 between the water inside and the water outside is 

 still better, since it is effected through all the available 

 sides of the cubic cage ; the results are the same, 

 and the animal inside the cage remains much smaller 

 than the one outside it. This series of experiments 

 answers the objections which might be raised on the 

 ground that in the smaller volume of water the 

 proportion of waste products might be larger, and 

 exert a noxious influence. But these waste products 



