. Bulletin of the Philosophical Society. Philosophical Society (University of Virginia). Scientific Section; Science; Philosophy. STUDIES IN HUMAN HEREDITY 299. o- â¢-i-D 6-H5i -n ^ â¢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 (, if dexterity is already deter- mined in the fertilized oviun) then either identical twins should be both right-handed or left-
. Bulletin of the Philosophical Society. Philosophical Society (University of Virginia). Scientific Section; Science; Philosophy. STUDIES IN HUMAN HEREDITY 299. o- â¢-i-D 6-H5i -n ^ â¢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 (, if dexterity is already deter- mined in the fertilized oviun) then either identical twins should be both right-handed or left-handedâor, 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 â 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 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 co
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