130 The Animal Mind 



ularly sensitive region to touch stimuli. It is, according 

 to Hofer, the skin nerves that are affected by the slow 

 vibrations which Parker thought to be the proper stimulus 

 for the lateral-line organs, and in certain cases he demon- 

 strated that such stimuli were responded to when the lateral- 

 line organs had been destroyed. The true function of the 

 lateral-line organs Hofer finds to be that of response to 

 streaming movements in the water. A skin sensitive- 

 ness to currents would be of the greatest practical value in 

 guiding the fish's migrations. 



36. Hearing in Amphibia 



Emergence from the water, on the part of adult Amphibia, 

 is accompanied by disappearance of the lateral-line canals, 

 and consequently of whatever sensations these mediate. In 

 the frog, the ear has a tympanic membrane lying at the sur- 

 face of the head. A single bone, the columella, with one 

 end against this membrane, lies across the middle ear. The 

 internal ear is not essentially different in structure from that 

 of the fish; there is no cochlea. Yerkes has made an 

 interesting study of the reaction of frogs to sound. He 

 found that they occasionally " straightened up and raised the 

 head as if listening" when other frogs croaked or made a 

 splash by jumping into the water. To no other sound did 

 he get any apparent response, nor was it possible to make 

 frogs in their native habitat jump or show any uneasiness 

 by producing any sort of noise, so long as the experimenter 

 remained invisible. "Apparently," Yerkes says, "they 

 depend almost entirely upon vision for the avoidance of 

 dangers." It is of course highly improbable that an organ 

 should be adapted only to the reception of the croaking of 

 other frogs and the splash of water, and not to noises made 



