1 5 5 5.0 5 5 5.0 5.00
2 5 4 4.5 2 2 2.0 3.25
3 4 5 4.5 2 5 3.5 4.00
4 3 4 3.5 1 1 1.0 2.25
5 5 2 3.5 0 1 0.5 2.00
6 3 2 2.5 1 0 0.5 1.50
7 2 1 1.5 1 2 1.5 1.50
8 5 1 3.0 0 0 0 1.50
9 1 3 2.0 0 0 0 1.00
10 1 2 1.5 1 0 0.5 1.00
11 1 1 1.0 0 0 0.50
12 1 1 1.0 0 0 0.50
13 0 0 0 0 0 0
14 0 0 0 0
15 0 0 0 0
[Illustration: FIGURE 33.—Plasticity curves. In the left margin are given the indices of modifiability (the number of tests necessary for the establishment of a perfect habit). Below the base line the age of the individuals is given in months. Curve for males, ————; curve for females, - - - -; curve for both males and females,——. When these three plasticity curves are completed, they will represent the indices of modifiability as determined for ten individuals at the age of 1 month, and similarly for the same number of individuals at each of the ages, 4, 7, 10, 13, 16, and 19 months.]
The detailed results for the one-month old individuals appear in Table 43; those for the four-month individuals in Table 52. The general averages for the former are to be found in the third column of Table 46, under the heading "10 tests per day"; those for the latter in the last column of Table 52. Mere inspection of these tables reveals the curious sex difference which goes far towards justifying the presentation of this uncompleted work. The index of modifiability for the ten one-month individuals is 88 (that is, 88 tests were necessary for the establishment of a habit); for the four-month individuals it is 102.5. The heavy solid line of Figure 33 joins the points on the ordinates at which these values are located. Apparently, then, the dancer acquires the white-black discrimination habit less readily at the age of four months than at the age of one month.
Further analysis of the results proves that this statement is not true. When the averages for the two sexes are compared, it appears that the males learned much less quickly at four months than at one month, whereas just the reverse is true of the females. The dash and dot line of the figure extends from the index of modifiability of the one-month males (72) to that of the four-month males (120); and the regularly interrupted line similarly joins the indices of the one-month (104) and the four-month (85) females. In seeking to discover age differences in docility or ability to profit by experience we have stumbled upon what appears to be an important sex difference. Perhaps I should add to this presentation of partial results the following statement. Since there are only four individuals in the four-month group, two of each sex, the indices are not very reliable, and consequently too much stress should not be laid upon the age and sex differences which are indicated.
In view of this impressive instance of the way in which averages may conceal facts and lead the observer to false inferences, I wish to remark that my study of the dancer has convinced me of the profound truth of the statement that the biologist, whether he be psychologist, anthropologist, physiologist, or morphologist, should work with the organic individual and should first of all deal with his results as individual results. Averages have their place and value, but to mass data before their individual significance has been carefully sought out is to conceal or distort their meaning. Too many of us, in our eagerness for quantitative results and in our desire to obtain averages which shall justify general statements, get the cart before the horse.
Figure 33 presents the beginning of what I propose to call plasticity curves. When these three curves are completed on the basis of experiments with five dancers of each sex for each of the ages indicated on the base line of the figure, they will indicate what general changes in plasticity, modifiability of behavior, or ability to learn (for all of these expressions have been used to designate much the same capacity of the organism) occur from the first month to the nineteenth in the male and the female dancer, and in the race without respect to sex. So far as I know, data for the construction of plasticity curves such as I hope in the near future to be able to present for the dancing mouse have not been obtained for any mammal.
At present it would be hazardous for me to attempt to state any general conclusion concerning the relation of docility to age.
The initiative of the dancer certainly varies with its age. In scope the action system rapidly increases during the first few months of life, and if the animal be subjected to training tests, this increase may continue well into old age. The appearance of noticeable quiescence does not necessarily indicate diminished initiative. Frequently my oldest mice have shown themselves preëminent in their ability to adjust their behavior to new conditions. However, I have not studied individuals of more than eighteen months in age. One would naturally expect initiative to decrease in senility. All that I can say is that I have seen no indications of it.
We may now briefly consider the principal sex differences which have been revealed by the experiments. In sensitiveness I have discovered no difference, but it should be stated that no special attention has been given to the matter. In docility the males usually appeared to be superior to the females. This was especially noticeable early in my visual discrimination tests. The males almost invariably acquired a perfect habit quicker than the females. I may cite the following typical instances. Number 14 acquired the black-white habit with 40 tests; No. 13, with 60 (Table 10, p. 109). Of the five pairs of individuals whose records in white-black training appear in Table 43, not one contradicts the statement which has just been made. It is to be noted, however, that under certain conditions of training, for example, 20 tests per day, the female is at an advantage. Recently I have with increasing frequency obtained measures of docility which apparently favor the female. That this difference in the results is due to a difference in age is probable.
In labyrinth tests the female is as much superior to the male as the male is to the female in discrimination tests. From the tables of Chapter XIII I may take a few averages to indicate the quantitative nature of this difference. A degree of proficiency in labyrinth B attained by the males after 7.0 trials was equaled by the females after 6.2 trials. In labyrinth C the males acquired a habit as a result of 18.7 trials; the females, as a result of 13.8. And similarly in labyrinth D, 6.1 trials did no more for the males than 5.9 did for the females.