RELATIVE POTENCY OF HEREDITY AND ENVIRONMENT 535 



ence especially to such minute details as finger prints. He showed 

 that in some cases the resemblance between the palm and sole patterns 

 of twins was startlingly close, but that in other cases, although the 

 twins appeared to be duplicate twins, there was not much resemblance 

 between their finger-print patterns. One cannot but be impressed 

 with Wilder's feelings of uncertainty as he attempts to classify certain 

 pairs. The following extract makes my point clear. Speaking of 

 Case VII, Wilder says: 



"This case caused me considerable trouble owing to the precon- 

 ceived 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 ac- 

 quaintance did not seem as great The case is plainly one of 



fraternal twins that resemble one another somewhat more than the 

 average." 



This is t)Apical of the method used in gathering statistics on this 

 problem of twins and the relative values of nature and nurture as 

 factors in development. We assume that twms that come from one 

 egg are nearly identical and that two-egg twins are no more alike than 

 ordinary brothers and sisters. From this we conclude that heredity 

 is almost the sole factor in determining the character of the individual. 

 Then we infer that strikingly similar twins are one-egg and those less 

 similar are two-egg derivatives. We assume that identical heredity 

 should give identical characters and then turn around and assume that 

 identical individuals, and only those, have identical heredity, while 

 individuals that fall considerably short of identity have unlike heredity. 

 This is reasoning in a vicious circle with a vengeance. If we find that 

 twins are sufficiently alike to meet our preconceived ideas of what one- 

 egg twins ought to be, we class them as duplicate twins. If we find 

 that they are less alike than we think they ought to be, we class them 

 as fraternal twins. Unless we can be certain at the outset whether 

 twins are of the one-egg or the two-egg types, no safe conclusions can 

 possibly be based upon their degrees of resemblance or difference. 



The only way in which one could at all safely determine whether 

 twins have the same or different heredity is to observe the placental 

 connections of the twins at birth. Authorities such as Spaeth and 

 Schatz agree that one-egg twins are attached to a single placenta, while 

 two-egg twins have separate placentas which may be more or less fused, 

 but even when fused show no intercommunication of placental blood 



