100 ROSS G. HARRISON 
will of course be possible to account for its reversal by rotation of 
the elements about the proper axis. As an alternative to the 
rotation hypothesis, we might, however, consider reversal as due 
to an interchange in position of two of the determining groups in 
the elementary units (p. 89, fig. 136). In case of differentiation 
on all three of the axes, i.e., if the units themselves are asymmet- 
ric, then reversal could take place only in the latter way, unless 
it occurs altogether independently of the intimate structure. 
There is an analogy for reversal of this kind in the change of 
the asymmetry of organic molecules of known composition, as, 
for instance, in the Walden inversion by means of successive sub- 
stitutions, or in the conversion of dextrotartaric acid into racemic 
acid, by which transformation half of the dextrorotating groups 
are changed into the laevo form. Of course, these examples are 
mere analogies. 
Such questions have been touched upon by many of those who 
have studied twins and double monsters, but, unfortunately, 
the evidence both as to the cause and as to the occurrence of 
reversal of asymmetry is conflicting. In the case of human dupli- 
cate twins, it is certain that there is no situs inversus viscerum, 
except very rarely, and apparently even in double monsters the 
degree of fusion of the two individuals must be considerable for 
the asymmetry of the internal organs — heart and alunentary 
canal — to be reversed. On the other hand, it has been shown 
by Wilder ('04) that in duplicate twins the friction-skin patterns 
of the two mates may show mirror imaging, particularly those 
on the index fingers. A similar condition has been found by 
Newman ('16) in his study of variation of the scutes in armadillo 
quadruplets, except that here the matter is further complicated 
by the relation, in pairs, of the four individuals of a litter. ^^^ 
1" "Now in the armadillo there are many definite evidences of a system of 
symmetry common to all of the quadruplets, upon which has been superimposed 
a secondary symmetry system between twins. This in turn is more or less com- 
pletely obliterated later by a tertiary symmetry between the antimeric halves of 
the single individuals. In some sets evident traces of the primary system of 
symmetry persist as mirror-image relations between individuals of opposite 
pairs, but it is more usual to find no trace of the primary system. The secondary 
mirror-imaging between pairs is far more commonly in evidence, but is frequently 
