LIMITS OF HEREDITARY CONTROL 857 
‘‘(3) Differences occur more frequently on the left side.” 
In his concluding paragraph, here quoted, are given in concise 
form the chief results and conclusions derived from his studies 
of duplicate twins: 
The influence of the germ-plasm and its mechanism (i.e., the direct 
control exercised by heredity) is exerted upon the friction-skin surfaces 
only so far as concerns the general configuration, i.e., the main lines, the 
patterns and other similar features; the individual ridges and their 
details (minutiae) are apparently under the control of individual me- 
chanical laws to which they are subjected during growth. Have we then 
arrived at the limit of the control of the predetermining mechanism beyond 
which mechanical laws are alone operative; and is it then possible to hold 
that the modifications in this latter field are the results of individual expe- 
rience, and that they are similar in the various members of a given species 
solely because of similar environment? To these and similar questions 
we can give no answer at present; yet it seems likely that in the general 
subject of palm and sole markings, not only in man but in other mam- 
mals as well, we have a set of easily observed and very significant data 
which may yield important results to future investigators. 
In addition to his data on palm and sole patterns Wilder fur- 
nishes us with rather elaborate physical measurements of four 
sets of duplicates. He realizes, however, that dimensional data 
are ‘‘far less determinative than are other characters employed, 
since they are liable to fluctuations through numerous causes, 
both internal and external, and it could hardly be expected that 
the similarities here would be very striking.’”’ Yet some most 
striking physical resemblances have been brought out by different 
authors. Vernon (’03), for example, gives the data for two pairs 
of identical twins, one of which, aged twenty-three, showed an 
average per cent difference of 0.28 per cent; while the other, 
aged twelve, a somewhat greater difference of 0.71 per cent. 
Weismann presents the data on one pair of twin brothers, aged 
seventeen, who showed a per cent difference of 2.2, nearly ten times 
that of Vernon’s first pair and about three times that of his second. 
Wilder’s measurements include a much wider range of charac- 
ters than do the others cited and are therefore probably of some- 
what greater value. Table 1 presents a brief summary of his 
data. 
