A great variety of sounds, ranging in pitch from a low tone in imitation of the bull frog’s croak to a shrill whistle, and in loudness from the fall of a pebble to the report of a pistol, were tried for the purpose of testing their effects upon the animals in their natural environment.  To no sound have I ever seen a motor response given.  One can approach to within a few feet of a green frog or bull frog 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.  I have repeatedly crept up very close to frogs, keeping myself screened from them by bushes or trees, and made various sounds, but have never succeeded in scaring an animal into a motor response so long as I was invisible.  Apparently they depend almost entirely upon vision for the avoidance of dangers.  Sounds like the splash of a plunging frog or the croak or pain-scream of another member of the species serve as warnings, but the animals do not jump into the water until they see some sign of an unusual or dangerous object.  On one occasion I was able to walk to a spot where a large bull frog was sitting by the edge of the water, after the frogs about it had plunged in.  This individual, although it seemed to be on the alert, let me approach close to it.  I then saw that the eye turned toward me was injured.  The animal sat still, despite the noise I made, simply because it was unable to see me; as soon as I brought myself within the field of vision of the functional eye the frog was off like a flash.

Many observers have told me that frogs could hear the human voice and that slight sounds made by a passer-by would cause them to stop croaking.  In no case, however, have such observers been able to assert that the animals were unaffected by visual stimuli at the same time.  I have myself many times noticed the croaking stop as I approached a pond, but could never be certain that none of the frogs had seen me.  It is a noteworthy fact that when one frog in a pond begins to croak the others soon join it.  Likewise, when one member of such a chorus is frightened and stops the others become silent.  This indicates that the cessation of croaking is a sign of danger and is imitated just as is the croaking.  There is in this fact conclusive evidence that the animals hear one another, and the probability is very great that they hear a wide range of sounds to which they give no motor reactions, since they do not depend upon sound for escaping their enemies.

The phenomenon of inhibition of movement in response to sounds which we have good reason to think the frogs hear, and to which such an animal as a turtle or bird would react by trying to escape, is thus shown to be common for frogs in nature as well as in the laboratory.  This inhibition is in itself not surprising, since many animals habitually escape certain of their enemies by remaining motionless, but it is an interesting