RESISTANCE OF FISHES. 337 



seen that the presence of the larger amount of oxygen increased 

 the resistance of the fishes to the high carbon dioxide. Hill and 

 Flack ('io) report this same antagonistic reaction between the 

 two gases and decide from some rather conclusive experiments 

 that the partial pressure of oxygen influences both the higher 

 and the lower limit of carbon dioxide which can be endured by 

 the organism. 



A comparison of the second and fourth columns of the table 

 will show that the fishes died more quickly in water that was 

 slightly alkaline than they did in water that was slightly acid. 

 The exact meaning of this difference is not clear. It may be 

 that the alkaline water has some detrimental local effect upon 

 the gills, or it may be that the neutrality mechanism (Henderson, 

 '13) of the fishes is more quickly affected for the worse in the 

 presence of IOW T oxygen, when the water is slightly alkaline than 

 when it is acid. At any rate the results indicate that the fishes 

 have a carbon dioxide optimum. This agrees with Bottazzi's 

 investigations for tissues, as quoted by Jerusalem and Starling 

 fio). They quote him as saying that "possibly a certain partial 

 pressure of carbon dioxide in the liquid or blood, bathing the 

 tissues, may represent the most favorable condition for the 

 exhibition of the tissues' activity, and that in every tissue it 

 ought to be possible to find an optimum tension of carbon 

 dioxide at which the tissue would do its best work." 



A comparison of the different columns in Table IV. will show 

 that the more effective factor in producing death was the low 

 oxygen. One good illustration of this will be noticed in the case 

 of the black bullhead (Ameiiinis melas] which occurs in columns 

 one and four. In these columns are presented results for two 

 sets of conditions, which differed only in the concentration of 

 the carbon dioxide, the other factor being low oxygen in both 

 cases. It will be seen that the bullhead lived nearly as long in 

 the high carbon dioxide experiment as it did in the low. This 

 indifference of the fishes to variations in carbon dioxide under 

 these conditions is not so clear with the other fishes, but it seems 

 to be generally true that o.i c.c. per liter of oxygen will produce 

 death sooner than 50 c.c. per liter of carbon dioxide. It should 

 be noted however (Shelford and Alice, '13) that most fishes 



