PHYSICAL CORRESPONDENCES IN 



TWO SETS OF DUPLICATE TWINS' 



Striking Degree of Identity Shown in Bodily Measurements and Skin Patterns 



of Palms and Soles 



Harris Hawthorne Wilder 

 Professor of Zoology, Smith College 



We are now, thanks to the long-con- 

 tinued researches of Newman and Pat- 

 erson, absolutely certain that all the 

 members of a single litter of young 

 armadillos come from what is originally 

 one egg, fertilized by one spermatozoon, 

 and that the striking similarities in cer- 

 tain details of the individual members 

 are due to the fact that all develop 

 equally from the same germ-plasm and 

 thus inherit the same characters. 



Human twins of the duplicate type^ 

 show precisely the same degree of 

 correspondence in similar parts, for the 

 friction-ridges of the palmar and plan- 

 tar surfaces, made from the fusion of 

 epidermal units (scales?) in rows 

 correspond morphologically very closely 

 to the rows of scales in the armadillo 

 carapace. It may thus be expected 

 a priori that the degree of correspond- 

 ence in the two . cases would be about 

 the same, and this, as nearly as can be 

 estimated by the inspection of many 

 cases in each, seems to be the case. 

 Both in normal sets of armadillos, and 

 in man, in the rather frequent cases of 

 true duplicates, we have undoubtedly 

 instances of typical polyeinbryony, with 

 remarkable correspondences in numer- 

 ous characters, more than is likely to 

 he found in single individuals from 

 separate eggs. 



Naturally on the basis of develop- 

 ment of twins from either a single egg 

 or from two separate ones, there must 

 be two and two only sharply defined 

 classes, with no possibility of inter- 

 mediate or transitional forms, and con- 

 sequently the presence of doubtful 

 cases, where, perhaps, neither the facial 



resemblance nor the palm and sole con- 

 figuration is absolutely conclusive, 

 threatens the security of the whole 

 theory. It is very encouraging, how- 

 ever, to hear from Newman a plea for 

 the classification of certain doubtful 

 cases formerly published by me as such, 

 under the head of true duplicates, 

 where the departures from identity are 

 only of about the same grade as those 

 occasionally met with among his arma- 

 dillos, where there is absolutely no 

 doubt that they are from a single egg, 

 and therefore duplicates. 



It is also quite possible, on the other 

 hand, that either fraternal twins, or 

 children of different births but with the 

 same parents, may show many similar 

 qualities, since very similar combina- 

 tions of character may chance to occur. 

 Indeed, with all the possibilities of link- 

 age, and with the consequent tendency 

 to the association of large groups of 

 characters, it would be unlikely that 

 such duplications did not occasionally 

 occur. One such case has come vmder 

 our direct observation, where there are 

 three sisters in a certain family, two of 

 them fraternal twins, and where the 

 younger single sister so closely resem- 

 bles one of the twins that she is gen- 

 erally considered by strangers to be 

 herself the other twin, while the latter 

 is of a quite difl^erent type, markedly 

 unlike the other two. Here, evidently, 

 is a case of three distinct eggs, two 

 born at the same time, and one at a 

 subsequent birth, but with a similar 

 composition in the germ plasm of this 

 latter and one of the two born simul- 

 taneously. 



^The term "duplicate," which the author uses throughout this article, has the same 

 meaning as the term "identical" used by the other authors writing in this issue. — Editor. 

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