STUDIES IN HUMAN HEREDITY 



299 





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•k 1 Identical TwlDS. 



Fig. 9. 



Chart, figure 9, includes a pair of identical twins one of which is left- 

 handed. Similar cases were described in my earlier paper. To that dis- 

 cussion should properly be added the following considerations : If the 

 factor for left-handedness is germinal (i.e., if dexterity is already deter- 

 mined in the fertilized oviun) then either identical twins should be both 

 right-handed or left-handed — or, one should always be left-handed and the 

 other right-handed. Although neither combination is the rule, the three 

 varieties probably occur (I have not yet seen a case where both were left- 

 handed). This evidence suggests that dexterity (left-handedness in this 

 case) is not unalterably established during the earliest developmental 

 stages, as is sex, for example. Such instances are disturbing to any heredi- 

 tary interpretation of left-handedness. It is not improbable, however, 

 when one recalls the several instances of. apparent reversal and imperfec- 

 tion of doDlinance, that the operation of later embryonic influences may be 

 the cause of such reversal and may account also for the dissimilarity in 

 dexterity of identical twins, originally but not unalterably determined. 



The ambidexterity of the young infant has its bearing on this point. 

 A baby under my observation handled her playthings indiscriminately and 

 equally well ■with either hand until the end of the sixth month. At that 

 time she showed a decided preference for the use of the right hand. At 

 the end of the first year she is completely right-handed.. We are dealing 

 here apparently with another of those numerous instances where the in- 

 dividual development recapitulates that of the race, primitive man having 

 been perhaps functionally to a large extent ambidextrous* as are the 

 anthropoid apes. Here again we come face to face, the foregoing assump- 

 tion being granted, -ndth the question of the inheritance of an acquired 



* Cunningham, loc. cit. 



