LIMITS OF HEREDITARY CONTROL 859 
a comparison of human identical twins, the fundamental weak- 
ness that it is based on an assumption which is clearly beyond the 
possibility of proof. Because certain twins exhibit a most strik- 
ing resemblance, it is assumed that they have been derived from 
a single fertilized egg. As Weismann (’02) expresses it, ‘“‘ Wir 
haben nun allen Grund, die erste Art von Zwillingen von zwer ver- 
schiedenen Hizellen abzuleiten, die letztere Art von einer einzigen, 
welche erst nach der Befruchtung durch eine Samenzelle sich in zwer 
Ever getheilt hat.” 
Were it possible in a number of cases to determine by exami- 
nation of the placental relationships of new-born twins whether 
they were duplicates or fraternals, and were it also possible to 
obtain data on these cases whose uterine history was known, we 
would have facts from which we could with confidence draw 
conclusions. Unfortunately, however, although monochorial 
twins have been observed at birth, no interest has been manifested 
in their resemblances. In view of the lack of satisfactory criteria 
‘as to the origin of the twins investigated, the writers on these 
subjects have reasoned backwards from the facts of resemblance 
to the assumption of common origin, a procedure far from safe, 
but doubtless justified by circumstances. An arbitrary criterion 
is thus set up for the classification of twins; and those that come 
up to specifications are classed as ‘duplicates,’ while those that 
fail to meet the arbitrary requirements are relegated to the rank 
of ‘fraternals.’ One cannot but be impressed, as he reads Wilder’s 
monograph, with the author’s feelings of uncertainty as he attempts 
to classify certain pairs. The following extracts indicate his 
attitude: 
No. VII. This casehascaused me considerable trouble, owing to the 
preconceived notion that the marks ought to be found identical. The 
family emphasized the facial resemblance of these twins and when I 
first saw them they certainly looked much alike. One was, however, 
an inch taller than the other, and the facial resemblance, after a short 
acquaintance did not seem as great. . . . The case is plainly one 
of fraternal twins that resemble one another somewhat more than the 
average. 
No. XIII. According to personal appearance these should be dupli- 
cates. I have never seen them, but the one who took the prints 
wrote: ‘The Misses*—— are so similar in coloring, figure and features 
