xvii THE SENSE ORGANS 339 



" it is never possible to get near to any frogs in the same 

 region after one has jumped in." The splash sound is sig- 

 nificant to them and puts them on their guard. 



On the other hand, many other sounds, varying greatly 

 in loudness and pitch, do not elicit any marked response. 

 " One may approach to within a few feet of a green frog or 

 a bullfrog and make all sorts of noises without causing it to 

 give any signs of uneasiness. Just as soon, however, as a 

 quick movement is made by the observer, the animal jumps. 

 . . . Sounds like the splash of a plunging frog, or the croak 

 or pain scream of another member of the same species, 

 serve as warnings, but the animals do not jump into the 

 water until they see some sign of an unusual or dangerous 

 object." 



It must not be inferred that frogs do not hear a great 

 variety of sounds simply because they give manifest signs 

 of attending to only a few sounds in which they have some 

 particular interest. Yerkes found that frogs which give no 

 other signs of perceiving sound, show a difference between 

 the rates of their respiratory movements before and after the 

 sound is made. The sound of a tuning fork, falling water, 

 a shrill whistle, the ringing of a bell, and other noises were 

 employed. Some of these produced little or no effect. The 

 shrill whistle and the ringing of a bell caused a decrease in 

 the rate of respiration, owing perhaps to fear, while the 

 sound of falling water caused the rate of respiratory move- 

 ments slightly to increase. The green frog (Rana clamitans) 

 was found to respond to sounds varying in pitch between 

 fifty and ten thousand vibrations per second. 



The reaction time of frogs to visual stimuli is also influ- 

 enced by sound. Yerkes found that frogs which were placed 

 in a glass aquarium so surrounded that the movements of the 

 observer could not be detected, would jump vigorously at 



