Part 7 (1/2)
Still another litter, which consisted of one female and four males, began to exhibit the quick, jerky movements, already mentioned, on the fourteenth day. On the morning of the fifteenth day three members of the litter definitely reacted to the tone of the steel bar, and also to the hammer blow when the bar was held tightly in the hand of the experimenter.
My observations were verified by another experimenter. Two individuals which appeared to be very sensitive were selected for special tests. Their reactions were obvious on the sixteenth, seventeenth, and eighteenth days; on the nineteenth day they were indefinite, and on the twentieth none could be detected. Some individuals of this litter certainly had the ability to hear for at least five days.
A sixth litter of four females and two males first gave indications of the change in behavior which by this time I had come to interpret as a sign of the approach of the period of auditory sensitiveness, on the seventeenth day. I had tested them almost every day previous to this time without obtaining evidence of hearing. The tests with the steel bar and the Galton whistle were continued each day until the end of the fourth week without positive results. To all appearances the individuals of this litter were unable to hear at any time during the first month of life.
Practically the same results were obtained with another litter of four females. The change in their behavior was obvious on the eighteenth day, but at no time during the first month did they give any satisfactory indications of hearing.
In the accompanying table, I have presented in condensed form the results of my auditory tests in the case of twelve litters of young dancers.
TABLE 5
PERIOD OF AUDITORY REACTION IN YOUNG DANCERS
PARENTS No. in Change in Ears Auditory Reactions Litter Behavior Open Appear Disappear
152+151 5 13th day 14th day 14th day 16th day 152+15l 8 (?) 13th day 13th day 17th day 152+151 5 13th day 13th day 13th day 17th day 152+151 4 10th day 12th day 13th day 15th day 410+415 5 14th day 15th day 15th day 19th day 410+415 6 13th day 14th day 14th day 18th day 420+425 2 12th day 14th day 14th day 17th day 210+215 5 17th day 13th day 17th day 19th day 210+215 6 11th day 14th day No reactions 220+225 6 16th day 14th day No reactions 220+225 6 17th day 13th day No reactions 212+211 4 15th day 14th day No reactions
Certain of the litters tested responded definitely to sounds, others gave no sign of hearing at any time during the first four weeks of life. Of the twelve litters for which the results of auditory tests are presented in Table 5, eight evidently pa.s.sed through an auditory period. It is important to note that all except one of these were the offspring of Nos.
151 and 152, or of their descendants Nos. 410 and 415 and Nos. 420 and 425. In fact every one of the litters in this line of descent which I have tested, and they now number fifteen, has given indications of auditory sensitiveness. And, on the other hand, only in a single instance have the litters born of Nos. 210 and 215, or of their descendants, given evidence of ability to hear.
These two distinct lines of descent may be referred to hereafter as the 400 and the 200 lines. I have observed several important differences between the individuals of these groups in addition to the one already mentioned. The 200 mice were sometimes gray and white instead of black and white; they climbed much more readily and danced less vigorously than those of the 400 group. These facts are particularly interesting in connection with Cyon's descriptions of the two types of dancer which he observed.
In criticism of my conclusion that the young dancers are able to hear certain sounds for a few days early in life, and then become deaf, it has been suggested that they cease to react because they rapidly become accustomed to the sounds. That this is not the case, is evident from the fact that the reactions often increase in definiteness during the first two or three days and then suddenly disappear entirely. But even if this were not true, it would seem extremely improbable that the mouse should become accustomed to a sudden and startlingly loud sound with so few repet.i.tions as occurred in these tests. On any one day the sounds were not made more than five to ten times. Moreover, under the same external condition, the common mouse reacts unmistakably to these sounds day after day when they are first produced, although with repet.i.tion of the stimulus at short intervals, the reactions soon become indefinite or disappear.
The chief results of my study of hearing in the dancer may be summed up in a very few words. The young dancer, in some instances, hears sounds for a few days during the third week of life. The adult is totally deaf. Shortly before the period of auditory sensitiveness, the young dancer becomes extremely excitable and pugnacious.
CHAPTER VII
THE SENSE OF SIGHT: BRIGHTNESS VISION
The sense of sight in the dancer has received little attention hitherto.
In the literature there are a few casual statements to the effect that it is of importance. Zoth, for example (31 p. 149), remarks that it seems to be keenly developed; and other writers, on the basis of their observation of the animal's behavior, hazard similar statements. The descriptions of the behavior of blinded mice, as given by Cyon, Alexander and Kreidl, and Kis.h.i.+ (p.47), apparently indicate that the sense is of some value; they do not, however, furnish definite information concerning its nature and its role in the daily life of the animal.
The experimental study of this subject which is now to be described was undertaken, after careful and long-continued observation of the general behavior of the dancer, in order that our knowledge of the nature and value of the sense of sight in this representative of the Mammalia might be increased in scope and definiteness. The results of this study naturally fall into three groups: (1) those which concern brightness vision, (2) those which concern color vision, and (3) those which indicate the role of sight in the life of the dancer.
Too frequently investigators, in their work on vision in animals, have a.s.sumed that brightness vision and color vision are inseparable; or, if not making this a.s.sumption, they have failed to realize that the same wave-length probably has markedly different effects upon the retinal elements of the eyes of unlike organisms. In a study of the sense of sight it is extremely important to discover whether difference in the quality, as well as in the intensity, of a visual stimulus influences the organism; in other words, whether color sensitiveness, as well as brightness sensitiveness, is present. If the dancer perceives only brightness or luminosity, and not color, it is evident that its visual world is strikingly different from that of the normal human being. The experiments now to be described were planned to show what the facts really are.
[Ill.u.s.tration: Figure 14.--Discrimination box. _W_, electric-box with white cardboards; B, electric-box with black cardboards. Drawn by Mr. C.H.
Toll.]
As a means of testing the ability of the dancer to distinguish differences in brightness, the experiment box represented by Figures 14 and 15 was devised. Figure 14 is the box as seen from the position of the experimenter during the tests. Figure 15 is its ground plan. This box, which was made of wood, was 98 cm. long, 38 cm. wide, and 17 cm. deep, as measured on the outside. The plan of construction and its significance in connection with these experiments on vision will be clear from the following account of the experimental procedure. A mouse whose brightness vision was to be tested was placed in the nest-box, A (Figure 15). Thence by pus.h.i.+ng open the swinging door at _I_, it could pa.s.s into the entrance chamber, _B_. Having entered _B_ it could return to _A_ only by pa.s.sing through one of the electric-boxes, marked _W_, and following the alley to _O_, where by pus.h.i.+ng open the swing door it could enter the nest-box. The door at _I_ swung inward, toward _B_, only; those at _O_, right and left, swung outward, toward _A_, only. It was therefore impossible for the mouse to follow any other course than _A-I-B-L-W-E-O_ or _A-I-B-R-W-E-O_. The doors at _I_ and _O_ were pieces of wire netting of 1/2 cm. mesh, hinged at the top so that a mouse could readily open them, in one direction, by pus.h.i.+ng with its nose at any point along the bottom. On the floor of each of the electric-boxes, _W_, was an oak board 1 cm. in thickness, which carried electric wires by means of which the mouse could be shocked in _W_ when the tests demanded it. The interrupted circuit const.i.tuted by the wires in the two electric-boxes, in connection with the induction apparatus, _IC_, the dry battery, _C_, and the hand key, _K_, was made by taking two pieces of No. 20 American standard gauge copper wire and winding them around the oak board which was to be placed on the floor of each electric-box. The wires, which ran parallel with one another, 1/2 cm.
apart, fitted into shallow grooves in the edges of the board, and thus, as well as by being drawn taut, they were held firmly in position. The coils of the two pieces of wire alternated, forming an interrupted circuit which, when the key _K_ was closed, was completed if the feet of a mouse rested on points of both pieces of wire. Since copper wire stretches easily and becomes loose on the wooden base, it is better to use phosphor bronze wire of about the same size, if the surface covered by the interrupted circuit is more than three or four inches in width. The phosphor bronze wire is more difficult to wind satisfactorily, for it is harder to bend than the copper wire, and it has the further disadvantage of being more brittle. But when once placed properly, it forms a far more lasting and satisfactory interrupted circuit for such experiments as those to be described than does copper wire. In the case of the electric-boxes under consideration, the oak boards which carried the interrupted circuits were separate, and the two circuits were joined by the union of the wires between the boxes. The free ends of the two pieces of wire which const.i.tuted the interrupted circuit were connected with the secondary coil of a Porter inductorium whose primary coil was in circuit with a No. 6 Columbia dry battery. In the light of preliminary experiments, made in preparation for the tests of vision, the strength of the induced current received by the mouse was so regulated, by changing the position of the secondary coil with reference to the primary, that it was disagreeable but not injurious to the animal. What part the disagreeable shock played in the test of brightness vision will now be explained.
[Ill.u.s.tration: FIGURE 15.--Ground plan of discrimination box. _A_, nest- box; _B_, entrance chamber; _W,W_, electric-boxes; _L_, doorway of left electric-box; _R_, doorway of right electric-box; _E_, exit from electric- box to alley; _I_, swinging door between _A_ and _B_; _O_, swinging door between alley and _A_; _IC_, induction apparatus; _C_, electric cell; _K_, key in circuit.]
An opportunity for visual discrimination by brightness difference was provided by placing dead black cardboard at the entrance and on the inside of one of the electric-boxes, as shown in Figure 14, _B_, and white cardboard similarly in the other box. These cardboards were movable and could be changed from one box to the other at the will of the experimenter. The test consisted in requiring the mouse to choose a certain brightness, for example, the white cardboard side, in order to return to the nest-box without receiving an electric shock. The question which the experimenter asked in connection with this test really is, Can a dancer learn to go to the white box and thus avoid discomfort? If we a.s.sume its ability to profit by experience within the limits of the number of experiences which it was given, such a modification of behavior would indicate discrimination of brightness. Can the dancer distinguish white from black; light gray from dark gray; two grays which are almost of the same brightness? The results which make up the remainder of this and the following chapter furnish a definite answer to these questions.