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104 Dr John Struthers on Variation in the Number — 
cause which determined the occurrence in one child might 
equally determine it in another, or it might be held that the 
first case having occurred spontaneously, the circumstance 
had operated by an impression on the mind of the mother. 
In all the cases of original variation, except in Case 1, and 
in one of the children in Dr Strachan’s case, it appeared on 
one limb only. 
This was not to any marked extent on one side of the 
body more frequently than on the other. The greater 
acquired strength of the right hand and foot might be sup- 
posed to operate on the offspring, but in the above cases it 
so happens that it appeared rather more frequently on the 
left side. Throughout the animal kingdom, when one side 
is more developed than the other, the right side is not 
selected more frequently than the left.* 
The variation begins more frequently on the hand than on 
the foot. In the non-hereditary group of cases in which it 
affected either a hand or a foot only, it appeared on the hand 
in eleven of the cases, on the foot in four cases. The here- 
ditary cases, by multiplying the fact, show a still higher 
proportion affecting the hand than the foot ; and in the cases 
in which it occurs on three of the limbs, it is on a foot more 
often than on a hand that the sixth digit is wanting. None 
of the cases show hereditary transmission of the peculiarity 
on the feet alone, while the cases of hereditary transmission 
on the hands only are more numerous than those showing 
it on the hands and feet. This greater frequency of its 
appearance on the hand may be connected with the zoologi- 
cal fact that in those mammals in which the digits differ in 
number on the two feet, the greater number is on the fore 
foot ; or, more generally, with the fact that greater separate 
use is made of the digits on the hand than on the foot. 
The additional digit appears nearly equally on the outer 
or on the inner side of the limb. In the non-hereditary 
group, reckoning the first case as one, it appeared in 7 cases 
on the outer side, and in 7 cases on the inner side, as an 
family of 8 brothers and 3 sisters, 8 brothers and one sister were born with 
digital increase. Two had an additional outer toe, one an additional outer 
finger, one had it on all the four limbs. It was unknown in the family as far 
back, at least, as the great-grand-parents. 
* See a paper by the author in the “ Edin, Medical Journal,” June 1863, 
