Records of the direction of whirling of one hundred individuals have been obtained. For twenty of these mice the determination was made by counting the number of complete turns in five-minute intervals at six different hours of the day. For the remaining eighty individuals the direction was discovered by observation of the activity of the animals for a brief interval at five different times. Naturally, the former results are the more exact; in fact, they alone have any considerable quant.i.tative value.

But for the problem under consideration all of the determinations are sufficiently accurate to be satisfactory.

The distribution of the individuals which were examined as to direction of whirling is as follows.

RIGHT WHIRLERS LEFT WHIRLERS MIXED WHIRLERS TOTAL

Males 19 19 12 50 Females 12 23 15 50

The frequency of occurrence of left whirlers among the females is unexpectedly high. Is this to be accounted for in terms of inheritance? In my search for an answer to this question I followed the whirling tendency from generation to generation in two lines of descent. These two groups of mice have already been referred to as the 200 line and the 400 line. The former were descended from Nos. 200 and 205, and the latter from Nos. 152 and 151. Individuals which resulted from the crossing of these lines will be referred to hereafter as of mixed descent. There were some striking differences in the behavior of the mice of the two lines of descent. As a rule the individuals of the 200 line climbed more readily, were more active, danced less vigorously, whirled less rapidly and less persistently, and were in several other respects much more like common mice than were the individuals of the 400 line. It is also to be noted (see Table 5) that few of the litters of the 200 line exhibited auditory reactions, whereas almost all of the litters of the 400 line which were tested gave unmistakable evidence of sensitiveness to certain sounds.

These differences at once suggest the importance of an examination of the whirling tendency of each line of descent.

The results for the several generations of each line which I had opportunity to examine are unexpectedly decisive so far as the question in point is concerned.

INDIVIDUALS OF THE 200 LINE

MALES FEMALES

First generation No. 200, ? No. 205, ?

Second generation No. 210, Mixed whirler No. 215, Left whirler Third generation No. 220, Mixed whirler No. 225, Mixed whirler Fourth generation No. 230, Right whirler No. 235, Mixed whirler Fifth generation No. 240, Right whirler No. 245, Left whirler

INDIVIDUALS OF THE 400 LINE

MALES FEMALES

First generation No. 152, Left whirler No. 151, Left whirler Second generation No. 410, Left whirler No. 415, Right whirler Third generation No. 420, Left whirler No. 425, Left whirler

One line of descent exhibited no p.r.o.nounced whirling tendency; the other exhibited a strong tendency to whirl to the left. Are these statements true for the group of one hundred individuals whose distribution among the three cla.s.ses of whirlers has been given? In order to obtain an answer to this question I have recla.s.sified these individuals according to descent and direction of whirling.

INDIVIDUALS OF THE 200 LINE

RIGHT WHIRLERS LEFT WHIRLERS MIXED WHIRLERS TOTAL

Males 7 6 8 21 Females 5 8 8 21 12 14 16 42

INDIVIDUALS OF THE 400 LINE

RIGHT WHIRLERS LEFT WHIRLERS MIXED WHIRLERS TOTAL

Males 4 9 1 14 Females 6 9 4 19 10 18 5 33

INDIVIDUALS OF MIXED DESCENT

9 10 6 25

Three interesting facts are indicated by these results: first, the inheritance of a tendency to whirl to the left in the 400 line of descent; second, the lack of any definite whirling tendency in the 200 line; and third, the occurrence of right and left whirlers with equal frequency as a result of the crossing of these two lines of descent.

It is quite possible, and I am inclined to consider it probable, that the pure dancer regularly inherits a tendency to whirl to the left, and that this is obscured in the case of the 200 line by the influences of a cross with another variety of mouse. It is to be noted that the individuals of the 200 line were predominantly mixed whirlers, and I may add that many of them whirled so seldom that they might more appropriately be cla.s.sed as circlers.

THE INHERITANCE OF INDIVIDUALLY ACQUIRED FORMS OF BEHAVIOR

The white-black discrimination experiments which were made in connection with the study of vision and the modifiability of behavior were so planned that they should furnish evidence of any possible tendency towards the inheritance of modifications in behavior. The problem may be stated thus.

If a dancing mouse be thoroughly trained to avoid black, by being subjected to a disagreeable experience every time it enters a black box, will it transmit to its offspring a tendency to avoid black?

Systematic training experiments were carried on with individuals of both the 200 and 400 lines of descent. For each of these lines a male and a female were trained at the age of four weeks to discriminate between the white and the black electric-boxes and to choose the former. After they had been thoroughly trained these individuals were mated, and in course of time a male and female, chosen at random from their first litter, were similarly trained. All the individuals were trained in the same way and under as nearly the same conditions as could be maintained, and accurate records were kept of the behavior of each animal and of the number of errors of choice which it made in series after series of tests. What do these records indicate concerning the influence of individually acquired forms of behavior upon the behavior of the race?

TABLE 53

THE INHERITANCE OF THE HABIT OF WHITE-BLACK DISCRIMINATION

Number of Errors in Daily Series of Ten Tests

MALES FEMALES

SERIES FIRST SECOND THIRD FOURTH FIRST SECOND THIRD FOURTH GENERA- GENERA- GENERA- GENERA- GENERA- GENERA- GENERA- GENERA- TION TION TION TION TION TION TION TION

No. 210 No. 220 No. 230 No. 240 No. 215 No. 225 No. 235 No. 245

A 6 5 6 7 8 4 4 7 B 6 8 8 8 8 7 6 5

1 6 7 6 5 7 6 5 4 2 4 3 1 5 5 6 4 5 3 3 1 4 5 3 4 4 3 4 5 0 3 4 2 1 3 1 5 3 0 4 2 1 3 3 0 6 2 1 4 2 2 1 1 1 7 1 0 3 1 1 1 2 0 8 0 0 1 0 0 0 2 3 9 0 0 0 1 1 0 0 0 10 0 0 1 0 2 1 1 11 0 0 0 3 0 0 12 0 0 0 0 0 13 0 0 0 0 14 0

I have records for four generations in the 200 line and for three generations in the 400 line.[1] As the results are practically the same for each, I shall present the detailed records for the former group alone.

In Table 53 are to be found the number of errors made in successive series of ten tests each by the various individuals of the 200 line which were trained in this experiment. The most careful examination fails to reveal any indication of the inheritance of a tendency to avoid the black box.

No. 240, in fact, chose the black box more frequently in the preference series than did No. 210, and he required thirty more tests for the establishment of a perfect habit than did No. 210. Apparently descent from individuals which had thoroughly learned to avoid the black box gives the dancer no advantage in the formation of a white-black discrimination habit. There is absolutely no evidence of the inheritance of this particular individually acquired form of behavior in the dancer.

[Footnote 1: This experiment was interrupted by the death of the animals of both lines of descent.]