Part 24 (1/2)
INDIVIDUAL, AGE, AND s.e.x DIFFERENCES IN BEHAVIOR
All dancers are alike in certain important respects, but to the trained observer of animal behavior their individual peculiarities are quite as evident, and even more interesting than their points of resemblance.
Omitting consideration of the structural marks of individuality, we shall examine the individual, age, and s.e.x differences in general behavior, rapidity of learning, memory, and discrimination, which have been revealed by my experiments. Observations which bear on the subject of differences are scattered through the preceding chapters, but in no case have they been given sufficient prominence to force them upon the attention of those who are not especially interested in individual peculiarities. It has seemed worth while, therefore, to a.s.semble all the available material in this chapter for systematic examination and interpretation.
In the pages which follow, individual, age, and s.e.x peculiarities are discussed in turn. Within each of these three groups of differences I have arranged in order what Royce has appropriately named the facts of discriminating sensitiveness, docility, and initiative. Individuals of the same age and s.e.x no less than those which differ in s.e.x or age exhibit important differences in ability to discriminate among sense impressions (”discriminative sensitiveness”), in ability to profit by experience (”docility”), and in ability to try new kinds of behavior (”initiative”).
Individual differences in sensitiveness to visual, auditory, tactual, and olfactory stimuli have been revealed by many of my experiments. The brightness discrimination tests conclusively proved that a degree of difference in illumination which is easily detectable by one dancer may be beyond the discriminating sensitiveness of another. Both the tests with gray papers and those with the Weber's law apparatus furnished striking evidence of individual differences in the kind of visual sensitiveness which throughout this book has been called brightness vision. I suspect that certain of the differences which were observed should be referred to the experience of the individuals rather than to the capacity of the visual organs, for training improves visual discrimination to a much greater extent than would ordinarily be thought possible. To the truth of this statement the results of the Weber's law experiments with No. 51 bear witness. Likewise in color discrimination there are individual differences, examples of which may be discovered by the examination of the results given in Chapters IX and X.
No differences in auditory sensitiveness appeared in my adult dancers, for in none of them was there definite response to sounds, but among the young individuals differences were prominent. I may call attention to the data on this subject which Table 5, p. 89, contains. The mice in four out of twelve litters gave no indications of hearing any sounds that I was able to produce; the remaining individuals responded with varying degrees of sensitiveness. I made no attempt to measure this sensitiveness, but it obviously differed from mouse to mouse. I feel justified, therefore, in stating that the young dancers exhibit extreme individual difference in sensitiveness to sounds.
My observations of differences in sensitiveness to other forms of stimulation were made in connection with training tests, and although they are not quant.i.tative, I venture to call attention to them. Indeed, I am led by the results of my study of various aspects of the dancer's behavior to conclude that the race exhibits individual differences in discriminating sensitiveness to a far greater extent than do most mammals, not excepting man. The importance of this fact (for I am confident that any one who carefully examines the detailed results of the various experiments which are described in this book will agree that it is an established fact) cannot be overlooked. It alters our interpretation of the results of training, memory, heredity, and discrimination experiments, and it leads us to suspect that the dancing race is exceedingly unstable.
I do not venture to make comparison of my own observations of the dancer's sense equipment with those of Cyon, Rawitz, Zoth, and Kis.h.i.+, for the differences are too great in many instances to be thought of as other than species or variety peculiarities. It has seemed fairer to compare only individuals of the same breed, or, as I have done and shall continue to do throughout this chapter, of two lines of descent.
With respect to docility individual differences are prominent. We need only turn to the various tables of results to discover that in modifiability of behavior, in memory, in re-learning, not to mention other aspects of docility, dancers of the same s.e.x and age differed strikingly.
Let me by way of ill.u.s.tration cite a few cases of difference in docility.
Number 1000 learned to discriminate white from black more quickly and retained his habit longer than any other dancer with which I have experimented. I should characterize him as an exceptionally docile individual. Table 44 offers several examples. Numbers 403 and 407, though they were born in the same litter and were alike in appearance and in conditions of life, acquired the white-black habit with a difference in rapidity which is expressed by the indices of modifiability 50 and 100. In other words, it took No. 407 twice as long to acquire this habit as it took No. 403. Similarly the ladder-climbing tests revealed important individual differences in ability to profit by experience. In the tables of labyrinth tests (38, 39, 40) individual differences are too numerous to mention. It required forty-nine tests to establish in No. 50 a labyrinth-C habit which was approximately equal in degree of perfection to that which resulted from twenty-two tests in the case of No. 52. The figures in this and other instances do not exaggerate the facts, for repeatedly I have tested individuals of the same litter, the same s.e.x, and, so far as I could judge, of the same stage of development, and obtained results which differ as markedly as do those just cited. If s.p.a.ce limits permitted, I could present scores of similar differences in docility which the problem, labyrinth, and discrimination methods have revealed.
In examining the detailed individual results of the various tables for differences of this sort, it is important to bear in mind that s.e.x, age, and descent should be taken into account, for with each of them, as will be shown clearly later in this chapter, sensitiveness, docility, and initiative vary. I have therefore based my statements concerning individual differences in docility upon the results of comparison of mice of the same litter, s.e.x, and age. It is safe to say that human beings similarly selected for comparison do not exhibit greater differences in ability to profit by experience than did these dancing mice.
The facts concerning individual differences in initiative which I have discovered are not less definite than those of the preceding paragraphs.
From the beginning of my study of the dancer I observed that what one individual would readily learn of his own initiative another never learned. For example, in the ladder-climbing experiment No. 1000 distinguished himself for his initiative, whereas Nos. 4 and 5 never acquired the habit of escaping from confinement by using the ladder. I noticed, in this test of the animal's ability to learn, that while one individual would be scurrying about trying all ways of escape, investigating its surroundings, looking, sniffing, and dancing by turns, another would devote all its time to whirling, circling, or was.h.i.+ng itself. One in the course of its activity would happen upon the way of escape, the other by reason of the limited scope of its activity, not the lack of it, would fail hour after hour to discover even the simplest way of getting back to its nest, to food, and to its companions. Hundreds of times during the past three years I have noticed important individual differences in initiative in connection with the discrimination experiments. The swinging wire doors which one dancer learned to push open before he had been in the box five minutes, another might not become familiar with through his own initiative for hours or days. In fact, it was not seldom that I had to teach an individual to pa.s.s from one compartment to the other by gently pus.h.i.+ng him against the door until it opened sufficiently to allow him to squeeze through. Occasionally a mouse learned to pull the doors open so that he could pa.s.s through the openings in either direction with facility. This was a form of individual initiative which I had not antic.i.p.ated and did not especially desire, so I did not encourage its development, but, nevertheless, at least one fourth of the mice which I experimented with in the discrimination box learned the trick. The other three fourths, although they were used in the box day after day sometimes for weeks, never discovered that they might return to the nest-box by pulling the swing-door through which they had just pa.s.sed as well as by entering one of the electric-boxes.
Another indication of individual initiative in action appeared in the tendency of certain mice to climb out of the experiment boxes or labyrinths. It would have been extremely easy for any of the mice to escape from the labyrinths by scaling the walls of the alleys, for they were only 10 cm. in height, and when a dancer stood on its hind legs it could easily reach the top with its nose. But, strange though it will seem to any one who has not worked with the dancer, not more than one in ten of the animals which I observed made any attempt to escape in this manner.
They lacked initiative. That it was not due to a lack of the power to climb, I abundantly demonstrated by teaching a few individuals that a scramble in one corner meant easy escape from the maze of paths. I do not think any one of the mice was physically incapable of climbing, but I am confident that they differed markedly, not only in the willingness to try new modes of action, but in the readiness with which they could climb. I have already said that individuals differ noticeably in the scope of their activity. By this statement I mean that they try a varying number of kinds of activity. As in the case of men, so in mice, one individual will do a greater number of things in a few hours than another will in weeks or months. The dancers differ in versatility, in individual initiative, as do we, albeit not so markedly.
Important differences which may with certainty be described as age differences are not so obvious as are such marks of individuality as have been set forth in the preceding pages. I have noted few changes in discriminative sensitiveness, other than those with regard to auditory sensitiveness, which could be correlated with age. In certain instances adults appeared to be able to discriminate more accurately and more easily than young mice, but it is difficult to say whether this change belongs under sensitiveness or docility. I have not made an ontogenetic study of the senses, and I am therefore unable to describe in detail the course of their development and decline. Of one important fact I am certain, that discriminative sensitiveness increases up to a certain point with age and with training.
Differences in docility which are obviously to be correlated with age abound. In the prime of its life (from the second to the tenth month) the dancer is active, full of energy, quick to learn; in its senility (during the second year) it is inactive, but at times even more docile than during the period of greatest physical development. Frequently I have noticed in connection with labyrinth tests that individuals of the age of a year or more learn much more quickly than do individuals of the age of two or three months. But, on the other hand, I have contradictory observations, for now and then I obtained just the opposite result in experiments to test docility. Evidently this is a matter which demands systematic, quant.i.tative investigation. Casual observation may suggest conclusions, but it will not justify them.
Early in my investigation of the behavior of the dancer I conceived the idea of determining the relation of modifiability of behavior (docility) to age. The question which was foremost in my mind and for which I first sought an answer may be stated thus: can the dancer acquire a given habit with the same facility at different ages? Since the visual discrimination experiment seemed to be well suited for the investigation of this problem I planned to train, in the white-black discrimination experiment, five pairs of dancers at the age of one month, and the same number for each of the ages four, seven, ten, thirteen, sixteen, and nineteen months.[1]
[Footnote 1: I have not been able thus far to determine the average length of the dancer's life. The greatest age to which any of my individuals has attained is nineteen months.]
To test the same individuals month after month would be the ideal way of obtaining an answer to our question, but I could devise no satisfactory way of doing this. The effects of training last so long, as the results of the previous chapter proved, and the uncertainty of their entire disappearance is so serious, that the same training process cannot be used at successive ages. The use of different methods of training is even more unsatisfactory because it is extremely difficult to make accurate quant.i.tative comparison of their results. It was these considerations that forced me to attempt to discover the relation of docility to age by carrying out the same experiments with groups of individuals of different ages.
As my plan involved the execution of precisely the same set of tests with at least seventy individuals whose age, history, and past experience were accurately known, and of which some had to be kept for nineteen months before they could be trained, the amount of labor and the risk of mishap which it entailed were great. To make possible the completion of the investigation within two years, I acc.u.mulated healthy individuals for several months without training any of them. In March, 1907, I had succeeded in completing the tests for the age of one month, and I had on hand for the remaining tests almost a hundred individuals, whose ages ranged from a few days to eighteen months. Had everything gone well, the work would have been finished within six months. Suddenly, and without discoverable external cause, my mice began to die of an intestinal trouble, and despite all my efforts to check the disease by changing food supply and environment, all except a single pair died within a few weeks.
Thus ended a number of experiments whose final results I had expected to be able to present in this volume. However, the work which I have done is still of value, for the single pair of survivors have made possible the continuance of my tests with other individuals of the same line of descent as those which perished, and I have to regret only the loss of time and labor.
As I have on hand results for ten individuals of the age of one month, and for four individuals of the age of four months, it has seemed desirable to state the problem, method, and incomplete results of this study of the relation of modifiability to age. The indices of modifiability for these two groups of dancers differ so strikingly that I feel justified in persisting in my efforts to obtain comparable data for the seven ages which have been mentioned.
TABLE 52
PLASTICITY (RELATION or MODIFIABILITY TO AGE)