196 



THE AMERICAN NATURALIST. [Vol. XXXVII. 



first sight it might seem that these experiments proved conclu- 

 sively that the ear of the killifish is, if not the only organ of 

 hearing, at least the chief organ for that sense, it is nevertheless 

 conceivable that the reduction in the number of reactions shown 

 by the earless fishes may be due not to the loss of the ear as a 

 sense organ but to the severity of the operation that the fishes 

 have undergone. 



To determine how much weight should be given to this objec- 

 tion and at the same time to ascertain the part played by the 

 skin in these reactions, I tested ten fishes in which the skin had 

 been made insensitive but in which the ears were intact. In a 

 total of one hundred trials, ten on each fish, there were ninety- 

 four responses to sound and six failures; in other words, these 

 fishes, though they had undergone more severe operations than 

 the earless ones, reacted almost exactly as normal fishes did. 

 These observations placed beyond a doubt the conclusion that 

 the ears of the killifish are stimulated by the disturbances set up 

 by the vibrating bass viol string, and the only question we have 

 to answer before a final conclusion can be safely drawn is, 

 whether these disturbances are really sound vibrations. 



If the aquarium is observed closely when the bass viol string 

 is made to vibrate, not only will a sound be heard but the whole 

 aquarium will be seen to vibrate and ripples will be noticed pass- 

 ing over the surface of the water. Is it not possible that the 

 fishes respond to the motion of the aquarium as a whole or to 

 the disturbance indicated by the ripples rather than to the true 

 sound-waves ? To determine this point, I substituted for the 

 bass viol string an electric tuning fork that gave a tone of 128 

 vibrations per second. The fork was so placed that its base 

 could by a slight movement be brought into contract with the 

 sounding-board and thus, without jar or disturbance to the aqua- 

 rium, the sound could be delivered to the contained water. 



On testing fishes under these conditions, it was found that the 



while those with ears very usually did respond I, therefore, 

 believe that I am entirely justified in drawing the final conclu- 

 sion that the ears of the killifish are stimulated by sound-waves, 

 that is, that this fish hears. It will be recalled that in the 



