570 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



in the female localized deposits of fatty tissue, not found in the 



apes, which produced profound alterations in the general form of the 



body. 



RIGHT-HANDEDNESS. 



To one who considers what precisely it means to fix the attention 

 and attempt the performance of some delicately adjusted and precise 

 action it must be evident that one hand only can be usefully employed 

 in executing the consciously skilled part in any given movement. 

 The other hand, like the rest of the muscles of the whole body, can 

 be only auxiliary to it, assisting, under the influence of attention, 

 either passively or actively, in steadying the body or helping the 

 dominant hand. Moreover, it is clear that if one hand is constantly 

 employed for doing the more skilled work, it will learn to perform it 

 more precisely and more successfully than either would if both were 

 trained, in spite of what ambidextral enthusiasts may say. Hence 

 it happened that when nature was fashioning man the forces of 

 natural selection made one hand more apt to perform skilled move- 

 ments than the other. Why precisely it was the right hand that 

 was chosen in the majority of manldnd we do not know, though 

 scores of anatomists and others are ready with explanations. But 

 probably some slight mechanical advantage in the circumstances of 

 the limb, or perhaps even some factor affecting the motor area of 

 the left side of the brain that controls its movements, may have 

 inclined the balance in favor of the right arm; and the forces of 

 heredity have continued to perpetuate a tendency long ago imprinted 

 in man's structure when first he became human. 



The fact that a certain proportion of mankind is left-handed, and 

 that such a tendency is transmitted to some only of the descendants 

 of a left-handed person, might perhaps suggest that one half of 

 mankind was originally left-handed and the other right-handed, 

 and that the former condition was recessive in the Mendelian sense, 

 or that some infinitesimal advantage may have accrued to the 

 right-handed part of the original community, wliich in time of stress 

 spared them in preference to left-handed individuals; but the whole 

 problem of why right-handedness should be much more common 

 than left-handed ness is still quite obscure. The superiority of one 

 hand is as old as mankind, and is one of the factors incidental to 

 the evolution of man. 



It is easily comprehensible why one hand should become more 

 expert than the other, as I have attempted to show; and the fact 

 remains that it is the right hand, controlled by the left cerebral 

 hemisphere, which is specially favored in this respect. This height- 

 ened educability of the (left) motor center (for the right hand) has 

 an important influence upon the adjoining areas of the left motor 



