Quantitative _versus_ qualitative results--Motives--Precautions-- Preference--Results of systematic habit-forming experiments--Curves of habit formation--Meaning of irregularity in curve--Individual differences--Comparison of curves for discrimination habits with those for labyrinth habits--Averages--The index of modifiability as a measure of docility--Reliability of the index. CHAPTER XV THE EFFICIENCY OF TRAINING METHODS Importance of measuring the efficiency of educational methods--Rapidity of learning and permanency of modifications wrought by training--Results of a study of the efficiency of discrimination methods--Comparison by means of indices of modifiability--Number of tests per series versus number of series--Efficiency as measured by memory tests. CHAPTER XVI THE DURATION OF HABITS: MEMORY AND RE-LEARNING Measures of the permanency of modifications in behavior--The duration of brightness and color discrimination habits--The relation of learning to re-learning--Can a habit which has been lost completely be re-acquired with greater facility than it was originally acquired?--Relation of special training to general efficiency--Does the training in one form of labyrinth aid the dancer in acquiring other labyrinth habits? CHAPTER XVII INDIVIDUAL, AGE, AND SEX DIFFERENCES IN BEHAVIOR Individual peculiarities in sensitiveness, docility, and initiative--The relation of docility to age--The individual result and the average--How averages conceal facts--Sex differences in docility and initiative-- Individual differences of motor capacity which seem to indicate varieties--Is the dancer pathological? CHAPTER XVIII THE INHERITANCE OF FORMS OF BEHAVIOR Characteristics of the race--Inheritance of the tendency to whirl in a particular way--Tests of the inheritance of individually acquired forms of behavior. INDEX ILLUSTRATIONS Dancing Mice--sniffing and eating _Frontispiece_ FIGURE 1. Color patterns of dancers. Record blanks 2. Double cage, with nest-boxes and water dishes 3. Double cages in frame 4. Photographs of dancers climbing (After Zoth) 5. Tracks of common mouse (After Alexander and Kreidl) 6. Tracks of dancer (After Alexander and Kreidl) 7. The inner ear of the rabbit (Retzius) 8. The membranous labyrinth of the ear of the dancer (After Rawitz) 9. Same 10. Same it. Model of the ear of the dancer (After Baginsky) 12. Ear of the dancer (After Kishi) 13. Ear of the dancer (After Kishi) 14. Discrimination box 15. Ground plan of discrimination box 16. Nendel's gray papers 17. Weber's law apparatus 18. Food-box apparatus 19. Waugh's food-box apparatus 20. Color discrimination apparatus 21. Ground plan of color discrimination apparatus 22. Cards for form discrimination 23. Labyrinth B 24. Labyrinth B on electric wires 25. Labyrinth A 26. Curves of habit formation for labyrinth B 27. Plan of labyrinth C, and path records 28. Labyrinth D