REACTIONS OF FISHES TO ATMOSPHERIC GASES 261 



fish studied, as was that of the fish studied by Duncan and 

 Hoppe-Seyler ('93), is clearly low. Fishes react to oxygen gradi- 

 ents, though usually indefinitely. 



On the other hand, the importance of carbon dioxide in fish 

 distribution has been largely overlooked. It is significant that 

 even in tap water, all the fish tried reacted very definitel}^ to 

 an amount of carbon dioxide that is scarcely greater than that 

 often found in ponds. Increased carbon dioxide is usually accom- 

 panied in nature by low oxygen and it is to the combination of 

 lack of oxygen (boiling) and increased carbon dioxide, that the 

 fish react most definitely. We accordingly feel justified in stat- 

 ing that the carbon dioxide content of the water (not excessively 

 alkaline) is the best single index (Shelford and Allee '12) of the 

 suitabi^;ty of water for fishes. Half bound carbon dioxide may 

 be of some importance in alkaline waters but our evidence tends 

 to show that in neutral or acid water it has little effect. Cer- 

 tainly in survey work designed to determine the suitability of 

 water for fishes, the determination of the carbon dioxide content 

 and the study of the conditions necessary for breeding (Shelford 

 '11) should not be omitted. 



From the standpoint of the physiology of fishes we have con- 

 tributed little but have added some confirming data to the obser- 

 vation that adult fishes are less sensitive than juvenile ones of 

 the same species, and that carbon dioxide acts as a narcotic and 

 in small quantities stimulates the respiratory center (Reuss '10). 

 The experiments indicate also that the fishes detect differences 

 in the character of water but the localization of the reception 

 of such stimuli has not been studied. 



From the standpoint of the behavior and psychology of fishes, 

 we note that fishes are able to react to stimuli by simple turnings 

 back, and that as a rule, they remain longer in water which does 

 not clearly influence the details of their activities. That the 

 formation of associations may enter into the latter type of reac- 

 tion and perhaps also into the former is suggested by the more 

 decided avoidance of treated water which comes with repeated 

 entrances into it. It seems, however, that such results may 

 possibly be otherwise interpreted. It is possible that time is 



