160 THE INFLUENCE OF INANIMATE SUKROUNDINQS. 



show that any other influence was impossible. Observations 

 do exist, on the contrary, which are calculated to warn us to bo 

 cautious in this matter. I will here only refer to the fact 1 

 myself observed that some Water-Lice [Asellus) which were 

 kept in an air-tight closed glass vessel for nearly two years, 

 and produced three or four generations, were, in the last 

 generation, abnormally small, though food, in the form of algee 

 and other plants, was constantly abundant, and the air above 

 the water, on opening the vessel, was found to be perfectly pure. 

 In this case lack of food was assuredly not the cause of the 

 small size of the Aselli; perhaps it was a result of constant 

 inbreeding, although in so small a number of generations — only 

 fovir — this is hardly probable. Hence it is a quite unfounded 

 assertion to say that the small size of animals in a small body 

 of water is always the result of a consequent deficiency of food, 

 since if this were so, whenever a more than sufficient supply of 

 food is at hand in the small body of water, the full growth 

 ought to be attained. But this is not always the case, which 

 proves that the often-observed effect of the volume of water 

 on the size of the creatures living in it is not, up to the present 

 date, understood, and still awaits an explanation. 



In order to solve this problem if possible, I carried out an 

 extensive series of experiments on the common pond-snail, 

 Lytnncea stagnalia. I selected this creature because its growth 

 is tolerably rapid in comparison with others, and because its long 

 spiral shell offers an excellent test, of which it is easy to avail 

 oneself in estimating its rate of growth. Moreover, this animal, 

 as I had learned from an accidental observation, is so remark- 

 ably sensitive to the effects of the volume of the water, that, 

 in the space of six days, the difference in the length of those 

 living in different volumes of water could be easily and 

 accurately determined. 



It will be understood that I can in this place give only the 

 general results of experiments carried on for more than two 

 yeajs. 



I instituted two series of experiments — one by separating the 

 animals from the same mass of eggs immediately they were 

 hatched, and placing them simultaneously in unequal bodies 



