RESISTANCE OF FISHES. 34! 



probably results in a decided increase in the oxidations of the 

 egg. This increase in the oxidations would tend to make the 

 eggs more sensitive to adverse conditions, especially in low 

 oxygen water, and it is very probable that eggs must at times 

 be subjected to severe conditions because of this. 



3 and 4. Resistance and Reactions of Young and Adult Fishes. 

 Of the importance of the resistance powers of fishes in general, 

 in waters where there is no escape from the harmful conditions, 

 there can be no doubt. Also the relative resistance of young and 

 adult fishes is a matter of consequence. In the experiments, 

 described in this paper, it was shown that concentrations of 

 detrimental factors, which are long in affecting adult fishes, may 

 prove fatal to small fishes of the same species, in a comparatively 

 short time. From this it will be seen that, ecologically and 

 economically, the fact that young fishes are more resistant per 

 unit of weight cannot be considered of great importance. 



With regard to the resistance of fishes to environmental com- 

 plexes it is possible to separate environments into two general 

 classes, namely, (i) environments whose conditions are constant 

 or nearly so from season to season and year to year (large bodies 

 of water) ; and (2) environments where the conditions fluctuate 

 considerably from season to season or from year to year (smaller 

 lakes and streams). 



If we consider fishes in their relation to these two types of 

 environments, with resistance as the factor in the foreground,. 

 w r e must look upon the matter of fish existence and distribution 

 as determined by the ability of fishes to withstand the constant 

 conditions in class I, or the varying conditions in class 2. This 

 then forces us to the conclusion that the matter of fish existence 

 in a given area is one of little flexibility. If, however, we con- 

 sider the behavior factor as important, it will be seen at once 

 that the matter of fish persistence is now determined by the 

 reactions of the fishes to the two sets of conditions, and becomes 

 at once flexible, because of the fact that the fishes may avoid 

 adverse conditions by moving out of them. 



That fishes will react in this way in natural waters is no 

 longer a question of doubt. Both adult and young fishes are 

 sensitive to detrimental concentrations of oxygen and carbon 



