It has been pointed out by Ballet that variations in the aptitude for left-hand mirror writing exist, especially in the case of those who cannot write without the aid of the visual image of letters. Since they copy this image in using the right hand for caligraphical purposes, they are tempted to do the same when the left is in use. In fact, the facility with which one learns mirror writing seems to depend on one's power of writing without recourse to these images. The explanation of the ease with which the left hand reproduces, in the guise of mirror writing, the movements of the other, is to be sought in the symmetrical arrangement of the muscles in relation round the body axis. Physiologists tell us, further, that the simultaneous contraction of two symmetrical muscles is more readily attained than that of two asymmetrical muscles. The law of symmetry and the law of least effort correspond. What is true of writing is no less true of all other forms of motor activity. In physical exercises the surest results are achieved by the synchronous contractions of symmetrical muscles, whereas education is much more arduous should this lesson from experience be ignored. For instance, nothing is easier than to make the arms describe circles in the same direction, but rotation in opposite directions is very difficult. Few people can revolve their thumbs in opposite ways. This is a matter of common observation among teachers of physical culture. The rapidity with which the action of swimming can be learned is in striking contrast to the slowness with which the art of fencing is apprehended. Little effort is required of the music beginner if his pianoforte exercises demand the activity of symmetrical muscles for their execution; on the other hand, the playing of a scale by the two hands in unison comes only with long practice, since it entails the simultaneous use of asymmetrical muscles. Facts such as these are of more than passing interest. One cannot afford to neglect their import where muscular education is concerned, whatever be its nature, whatever be its object. Yet there is an unfortunate tendency to concentrate attention on the development of the skill of one arm only, and that the right. Sometimes the use of the left arm for certain purposes is criticised adversely, and of course most people are congenitally less able to work with it. But habit, example, and even fashion, combine to render the right arm preponderant in everything, to the detriment of the other. It is a common occurrence to attribute awkwardness to this left arm, when its inferiority is really nothing else than a sign of faulty education. In many cases the left is as good as the right; its apparent _gaucherie_ is because of its attempt at executing movements which are similar to those of the right, instead of those which are correspondingly opposite.