ations on sense-organs, nerve cutting and like methods may aid us in determining what organ or portion of an organ is necessary for a particular function, but they can give us no trustworthy information concerning the relation existing between the senses and the normal behavior of the animal. Therefore, in the experiments now to be described, with the exception of those which were made to ascertain whether the ear is necessary for reaction to sounds, normal frogs were studied either in their native haunts or in the laboratory. All the detailed work was done with the green frog, Rana clamitans, but tests were made also with the leopard frog, R. pipiens, and the bull frog, R. catesbiana.

II.

REACTIONS OF FROGS IN NATURE TO SOUNDS.

My attention was first drawn to the subject of frog audition by failure to obtain motor reactions to sounds in an investigation of the time relations of the neural process of the green frog. Although a large number of sounds of different qualities, pitches and intensities were employed, no visible motor reactions were observed. This led me to seek the significance of what appeared to be either a surprising lack of sensitiveness to changes in the environment which would naturally be expected to stimulate the animal, or an interesting and important case of the inhibition of reaction to auditory stimuli. The question to be answered is, Are frogs deaf, or do they under certain conditions completely inhibit their usual reactions to sound?

Since they bear upon the question of deafness I quote the following observations on the influence of sounds in Nature from the auditory-reaction section of my earlier reaction-time paper.'

In order to learn how far fear and artificial conditions were causes of the inhibition of responses to sounds in the laboratory, and how far the phenomenon was indicative of the animal's inability to perceive sounds, I observed frogs in their native haunts.

By approaching a pond quietly, it is easy to get within a few yards of frogs sitting on the banks. In most cases they will not jump until they have evidence

'The Instincts, Habits and Reactions of the Frog. Harvard Psychological Studies, 1, 629-630 (Psychological Review Monograph, 4), 1903.

of being noticed. Repeatedly I have noted that it is never possible to get near to any frogs in the same region after one has jumped in. In this we have additional proof that they hear the splash-sound. To make sure that sight was not responsible for this on-guard condition in which one finds the frogs after one of their number has jumped into the water, I made observations on animals that were hidden from one another. The results were the same. I therefore conclude that the splash of a frog jumping into the water is not only perceived by other frogs in the vicinity, but that it is a peculiarly significant sound for them, since it is indicative of danger, and serves to put them on watch.'

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 uneasi ness. 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 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 sur prising, since many animals habitually escape certain of their enemies by remaining motionless, but it is an interesting phenomenon for the physiologist. We

have to inquire, for instance, what effects sounds which stimulate the auditory organs and cause the animal to become alert, watchful, yet make it remain rigidly motionless, have on the primary organic rhythms of the organism, such as the heart-beat, respiration, and peristalsis. It is also directly in the line of our investigation to inquire how they affect reflex movements, or the reaction time for any other stimulus-what happens to the reaction time for an electrical stimulus, for example, if a loud noise precede or accompany the electrical stimulus.

III. THE INFLUENCE OF SOUNDS ON REACTIONS TO OTHER STIMULI.

I. Influence of sound on respiration and visual reactions.— Observations described in my earlier paper prove that respiration is modified by sounds, and it was also noted that the attempts of a frog to seize a moving object are reinforced by the sound of a tuning fork.1

2. Influence of sounds on tactual reactions. —A more detailed study has been made of the influence of sounds on tactual reactions and of the significance of the temporal relations of the stimuli.

A reflex movement of the leg was chosen as an indication of the action of the stimuli and the influence of sounds was observed under the following conditions. A frog was placed on a saddle-like holder and kept in position by linen bands over the back and a wire screen cap over the head, as shown in Fig. 4. Under these conditions the hind legs usually hang free and limp and any movement which may be made by them in response to a stimulus can be read in millimeters from a scale, attached to the holder. This method of measuring the value of stimuli in terms of leg reflex has been used by several other investigators, most recently by MERZBACHER. In this connection it is of interest to note that neither MERZBACHER nor EWALD were able to detect movements of the leg of the frog in response to sounds. Even the croaking of another frog near by had no apparent effect.2

11. c. p. 634.

"MERZBACHER, L. Ueber die Beziehungen der Sinnesorgane zur den Reflexbewegungen des Frosches. Pflüger's Arch., 81, 254, 1900.

I found it desirable, as did MERZBACHER, to observe the movements of a shadow of the leg on the scale and thus read the amount of movement, rather than to watch the leg itself and attempt to project it upon the scale.

As is indicated in the figure the auditory and tactual stimuli were given automatically by means of a swinging pendulum (P) which was held in position by the magnet a until released by the experimenter. Early in its swing the pendulum turned the key m thus completing a circuit which caused the auditory stimulus to be given; later in the swing the key n was turned, and the tactual stimulus thus given through the magnetic release of the lever . The interval between the auditory and the tactual stimuli could be varied from. 1" to .9" by change in the position of the key n. For giving the two stimuli simultaneously a double hand key was employed.1

The auditory stimulus was either the sound of a quick hammer blow or the ringing of an electric bell for a certain interval. Figure 4 shows the bell. It was placed 80 cm. from the frog, and in order that the influence of vibrations might be avoided was suspended from the pendulum frame. When the hammer was used it was placed 60 cm. from the frog, on the pendulum table. The frog and the apparatus for tactual stimulation occupied a separate table which was not disturbed by the jars of the pendulum table. The tactual stimulus was given by a rubber cone, T, Fig. 4, 2 mm. in diameter at its apex. This rubber point, after the electric release of the lever to which it was attached, struck the frog at the middle point of a line drawn between the posterior margins of the tympana. The intensity of the stimulus could be varied by weighting the lever (see w in the figure).

Under the conditions of experimentation described above, a tactual stimulus regularly causes a reflex movement of the 1A full account of this method and the results of a study of the phenomena of auditory-tactual reinforcement and inhibition may be found in Pflüger's Archiv, Bd. 107, S. 207, 1905, under the title "Bahnung und Hemmung der Reac tionen auf tactile Reize durch akustiche Reize beim Frosche." In this connection I mention only such aspects of the investigation as bear on the subject of audition.