II SEMPER'S EXPERIMENTS 87 



do not require to be taken into account in such cases. 

 I have also seen that unless a given volume of water 

 has been inhabited for a long time or by a large 

 number, it exerts no bad influence on the growth of 

 other pond-snails. For instance, take two identical 

 vessels, and use, in one, one litre of pure fresh water, 

 in the other the same quantity of water in which a 

 pond-snail has been living two or three months ; in 

 each put one young Lymncza, of same age and brood ; 

 kill both after the same time (three or four months) ; 

 there is no observable difference. Of course, if the stale 

 water has been much inhabited by pond-snails, the 

 growth of the fresh ones is impaired. But such 

 impairment does not occur in my experiments, and I 

 do not well see how waste products could accumulate 

 more in a narrow-surfaced vessel where aeration is 

 very good, as the plants show, than in a large- surfaced 

 vessel, where it must be also good, the quantity of 

 water being equal in both, or even, as is the case in 

 many of my experiments, superior in the former. 1 



So it seems that Semper' s interpretation has to be 

 dismissed as unnecessary, and that a simpler expla- 

 nation is furnished by the results of my experiments 

 an explanation which depends upon known principles 



1 All these experiments shall be related in greater detail in a forth- 

 coming memoir, as soon as I have completed the experiments 

 which are yet being continued (March, 1892). 



