H. E. Jordan 75 



Chart Fig. 78 was given to me by Professor J. P. Campbell, of the 

 University of Georgia. It shows peculiarly well the inadequacy of only 

 the dii-ect ancestry in a study of hereditary lefthandedness and the 

 hereditary influence and significance of collateral ancestry, and suggests 

 tlie rarity of " pure " strains with respect of dexterity. Also, it is of 

 especial interest in giving a pair of duplicate twins, only one of which is 

 lefthanded (and both " cross-eyed "). This condition is the exception of 

 what usually obtains in the case of such twins. It suggests more forcibly 

 perhaps than any othei' fact the verity of " degi'ees of bias " to use of 

 hand. 



Chart Fig. 79 is supplied by Mr J. H. Green, of Clitlon Forge, 

 Virginia. It is a fau-ly complete five-generation history of a large 

 family in which appear both lefthandedness and a tendency to twinning. 

 Nine pairs of twins appear; one member of pair 5 is lefthanded; but 

 these are not identical twins. The other members of this fraternity are 

 another pair of twins ; and the lefthanded condition appears in the 

 expected ratio for the DR x DR cross. Coincidence of lefthandedness 

 and twinning in the same fraternity is frequent ; but no very likely 

 explanation suggests itself regarding causative relationship. The 

 explanation of lefthanded duplicate twins however lies most probably in 

 the ancestral presence of both conditions ; hereditary twinning would 

 then produce a lefthanded pair when the determiner for lefthandedness 

 was coincidently present in the case of a dujjlicate set (c£ Figs. 22 and 

 57). The complete absence of lefthandedness in the product of mating 

 ^ X X J' Y is interesting as suggesting a " pure " extraction from a 

 " tainted " stock. 



A summary of the 79 histories charted gives a proportion of 173 

 lefthanded males to 14.3 females. This result is the opposite of the one 

 usually recorded, namely, that females show a heavier incidence of left- 

 handedness. My earlier studies gave an approximate equality ; and the 

 discrepancy here noted is not sufficiently great to have significance as 

 contradicting the general conclusion that males and females are equally 

 " susceptible " to lefthandedness. 



Discussion. 



No attempt has been made in the foregoing to explain contradictions 

 and exceptions. In view of the present study, and of two earlier ones, 

 no one, I believe, will seriously dispute the conclusion that lefthandedness 

 is hereditary. That it follows in inheritance Mendelian principles, a 



