200 THE AMERICAN NATURALIST. [Vol. XXXVII. 



region where surface waves affect it, it will swim downward out 

 of the reach of these, though it does not go to the bottom of the 

 aquarium as a fish with lateral-line organs does. If it is placed 

 in a current of water, it will swim as vigorously against the cur- 

 rent as a normal fish will. Thus the skin, though not stimulated 

 by sound from a tuning fork or the slight inaudible trembling of 

 the whole mass of water, is stimulated by currents and by sur- 



Hence the three sets of sense organs under consideration 

 maybe regarded as having slightly different kinds of stimuli: 

 the skin being affected by surface waves and by currents; the 

 lateral-line organs by slight inaudible movements of the whole 

 mass of water; and the ears by the still more delicate vibrations 

 of water particles, sound. These three sets of sense organs, 

 therefore, are not only genetically connected in that they repre- 

 sent so to speak three generations of organs, but their evolution 

 has been toward more and more delicate means of stimulation. 

 From this standpoint the lateral-line organs of the fishes and the 

 amphibians may be said to be delicate organs of touch and even 

 the ear as an organ of hearing may be looked upon as an exqui- 

 sitely refined apparatus of much this same kind. 1 Hearing, then, 

 is a most delicate form of touching, and the organ of hearing has 

 developed late in the animal series because its processes are not 

 original but are derived from those of the more primitive sense, 

 touch. Many fishes possess at once the complete series of sense 

 organs leading from touch to hearing, and in these animals the 



very distinct things, a condition, however, that we can easily 

 understand, for we have lost the intermediate sense, that of the 



