Page:Popular Science Monthly Volume 44.djvu/630

614 patient could still understand tunes, and, further, could imagine tunes "in his head" (what the French call intérieur), while he could not sing them. This shows a close connection in locality between motor speech and motor music function, while a slight separateness of the two centers in locality in the left hemisphere, explains cases of motor aphasia in which musical execution is preserved. Further, Frankl-Hochwart declares that no cases are recorded of amusia from lesion in the right hemisphere, and Starr says (in a private letter) of a patient of his: "My patient is right-handed, and music does follow speech in being unilaterally located; . . . it is well proved that the musical faculty is onesided in location." Despite these positive opinions, I think more critical cases with autopsy are necessary to make the position quite secure.

All this means simply that the general cause to which is due the fact of right-handedness is also the cause, through further differentiation and emphasis in the same local seat, of the development of musical ability and of speech. It now remains to ask: What was or is this cause, and when in the race-history series did it begin to operate? There are only two hypotheses of any force—either "experience" or "spontaneous variation" at some stage in biological development.

It is extremely improbable that dextrality should have arisen among the quadrupeds (or amanous bipeds), for experience was lacking of unilateral stimulation, and a spontaneous variation of this kind would have produced such inconvenience of locomotion and ultimately such asymmetry of form that it would have been weeded out. As an extreme example, fancy a bird which is dextral in its flight.

As soon as we come to bipeds with hands, however, these reasons do not hold. Their locomotion does not depend on manual symmetry, and any dextrality, however slight, would be of direct advantage in climbing, fighting, breaking sticks, and pulling fruit; since a disproportionate growth of one side would give that side greater strength than either side would possess in animals of symmetrical development in the same environment. A very strong one-armed man can keep at bay a weaker man with two arms, or destroy him, and this is emphasized in animals, where brute force is the only resource. It is difficult to find, however, in the habits of simians any ground for believing that there has