REACTIONS OF FISHES TO ATMOSPHERIC GASES 255 



cases counterbalances the apparent time preference for one end. 

 To be consistent, there must be more turnings from the end 

 in which the least time is spent. When this does not occur, the 

 apparent time preference is neutralized to a greater or less 

 extent, as the case may be. The figures in the column marked 

 'rating' in table 19 are attempts to express numerically the inten- 

 sity of reaction when both turnings and time preferences are 

 considered. The ratings were obtained by adding the difference 

 in the percentage of time spent in the two ends, to the difference 

 in the percentage of total number of turnings. This sum divided 

 by two gives the rating average. In the experiments where the 

 fish avoided the tap water or the water nearest like that in which 

 they had been kept, they were rated as negative. Thus the rating 

 for Ameiurus from the low oxygen stock in relation to the boiled 

 water is in favor of the boiled water end of the experiment, so 

 they are rated as +18. 



In the experiments the degree of negative reaction to the vari- 

 ous factors and combinations of factors is shown in table 20. 

 The ratings of the boiled water, oxygen, et cetera, group are very 

 conservative, as the averages upon which they are based include 

 cases where fishes reacted by coming to the surface, et cetera, and 

 which neutralized the reaction in experiments where our methods 

 are effective (p. 246). In some cases few experiments were run 

 with a given set of conditions because the reactions were very 

 decided, so that further experiments were unnecessary for the 

 purpose of this paper. Considering the averages as they stand, 

 it will be noted that the greatest vigor of reaction is shown to 

 carbon dioxide in boiled water and to the combination of carbon 

 dioxide and boiled water with tap water at the other end of the 

 gradient. The reactions to carbon dioxide in tap water and that 

 to acetic acid in boiled water are about equal and stand next 

 in rank. Of the experiments where carbon dioxide and acetic 

 acid are not concerned, the reaction is most definite to an oxygen 

 gradient in boiled water. Fishes vary greatly in the vigor of 

 their reaction to both boiled water and to oxygen alone. 



Hybopsis proved most sensitive of all the fishes tried. The 

 graphs in chart 4 show the manner of reaction and the movements 



