MODIFICATION OF THE BEHAVIOR OF FISHES 23 



(Holmes, '11) to strike its nose against the glass partition, and 

 from the standpoint of the fishes in relation to nature, modifi- 

 cation by contact with stimulating water is of a more significant 

 type. 



Any explanation of the modification demands first a clear 

 statement of the problem. With figure 1 before us, this state- 

 ment together with a discussion of various possible explanations 

 will be made. One explanation that may be advanced is that 

 the fishes were depressed by the modified water and thus tended 

 to stop swimming forward on entering it and so finally came to 

 rest in the normal water. This is very clearly not the case, 

 although it is suggested by some of the graphs because the 

 cross movements of the fish are not indicated. With one ex- 

 ception, the modified water was stimulating. The exception 

 was a temporary depression of activity of the rock bass due to 

 lack of oxygen. In a few experiments, the rock bass entered 

 the boiled water and, being depressed, stayed there a half hour 

 or more. Finally, however, they began to move back and forth 

 and selected the tap water end. 



Another explanation is that the fishes are stimulated by the 

 modified water and thus move out of it more quickly and spend 

 more time in the untreated water. This is clearly what hap- 

 pened in many cases. The fishes rushed forward more rapidly 

 when they encountered the stimulating conditions, and upon 

 reaching the end, turned and moved out quickly. Still, this does 

 not explain the turning in the gradient which took place more 

 often than the simple acceleration. Neither does it explain 

 what was also sometimes true, namely, that fishes did not show 

 the acceleration until they had encountered the treated water 

 a number of times. 



A third explanation that may be advanced is this: When the 

 fishes had been exposed to the low oxygen water or to water 

 containing much carbon dioxide for a long enough time to affect 

 the oxygen or carbon dioxide content of much or all of the blood 

 and thus affect the nervous system as a whole, the fishes began to 

 turn back. The change in gas content caused a change in the 

 physiological state of the fishes, so that they were more sensitive 

 to the surrounding medium. In connection with this explana- 

 tion, and the preceding one as well, certain facts brought out in 

 the experiments should be noted. When dropped into water 



