248 MORRIS M. WELLS. 



The factors that regulate the distribution of the plankton 

 in the lakes are undoubtedly numerous. The only certain way 

 to determine them is to investigate experimentally the reactions 

 of the animals to the factors concerned, both singly and in com- 

 bination. To do this would be tedious but not especially dim- 

 cult. As an index to the distribution of these forms, I believe 

 that the presence and position of a neutral layer of water will be 

 found to be important. 



Besides the experimental data presented in the papers by 

 Birge and Juday, the literature contains much other experimental 

 evidence which bears directly upon the question of the toxicity 

 of neutrality to organisms. Much of this evidence is found in 

 connection with experiments upon the toxic effects of distilled 

 water, and the action of salts in antagonizing this toxicity. In a 

 series of papers published by Ringer and his students between 

 the years 1883 and 1893 the question of the toxicity of distilled 

 water was investigated and its reality apparently demonstrated. 

 It was also shown that various salts are effective in neutralizing 

 this toxicity, some being much more efficacious than others. 

 In 1893 Naegeli showed that for Algae (Spirogyra) at least, 

 the toxicity of distilled water was due to contamination from 

 the copper stills in which it was prepared. Locke ('95) con- 

 firmed Naegeli's results by showing the effect upon certain 

 fresh-water animals to be due also to the minute amounts of 

 copper present, and Ringer ('97) again taking up the subject 

 reversed his former conclusions and confirmed those of Locke. 

 Jennings ('97) found that Paramcecia live for weeks in distilled 

 water. Moore ('oo) says that young trout and tadpoles (unfed) 

 live as long in distilled as in tap water, i. e., several weeks. 

 Lillie ('oo) says that Planaria maculata will live in distilled water. 

 Pure distilled water seemed then not to be toxic to fresh-water 

 animals though apparently toxic to most marine animals. Fun- 

 dulus eggs seem to be an exception among marine animals 

 (Loeb, '99), as they can live in distilled water for weeks and 

 still produce normal embryos. In 1903 Bullot after testing the 

 effects of distilled water upon the fresh-water amphipod, 

 Gammarus concluded that pure distilled water was toxic to this 

 crustacean. Bullet's experiments were performed with great 



