REACTION AND RESISTANCE OF FISHES. 253 



Peters ('08) makes no mention of the possibility of the neu- 

 trality of the distilled water which he used, having something to 

 do with its toxicity, yet in a previous paper ('07) he recognizes 

 the importance of the presence of a certain concentration of 

 hydrogen ion for the existence of certain protozoa in hay in- 

 fusions. On page 346, he says: 'The data obtained indicate 

 that, of the chemical conditions, the concentration of acid ... is 

 one of the chief factors determining the biological content and 

 history of a culture." 



From the data and discussion that have gone before, it seems 

 certain that the chemical reaction of the water is a factor of 

 marked importance in the life history of fresh-water animals. 

 Some fresh-water forms are apparently positive to alkalinity as 

 seen in the fresh-water lakes (Birge and Juday, loc. cit.} and 

 others, that normally live in water that is acid with CO 2 are not 

 killed by living in alkaline water (isopods). On the other hand, 

 many forms, and probably most of the fresh-water fishes belong 

 here, are always found in acid water if such be available, and 

 these forms cannot live normally in neutral to alkaline water. 

 Shelford and Powers ('15) have shown that marine fishes select 

 the alkaline side of neutrality in a gradient, and in this dif- 

 ference in the behavior of the fishes, lies a key to the fundamental 

 physiological difference in the organisms of these two habitats. 

 Fresh-water fishes must live in the presence of an excess of hy- 

 drogen ion if their life processes are to be carried on in normal 

 fashion. Shelford ('14) states that the carbon dioxide content 

 of the water over the breeding grounds of fresh-water fishes 

 should not average more than I c.c. per liter, nor exceed 5 c.c. 

 during the summer months. This statement is probably wrong 

 in limiting the average to I c.c. per liter for some fishes, as the 

 green spotted sunfish and the crappies are negative to this 

 small concentration of CO 2 showing a preference for slightly 

 higher concentrations. Blue-gills, on the other hand, select a 

 degree of acidity that is very little above neutrality. The CO 2 

 concentration selected by fishes probably varies with the season, 

 and certainly with the salt content of the water in which they 

 live. The variations of the CO 2 optimum in salt concentration 

 will be discussed in the second paper of the series. 



