388 Duplicate Twins and Double Monsters 



of the same family; the second, those who are invariably of the same 

 sex and who otherwise so closely resemble one another that it is difficult 

 or impossible, especially during youth, for those not intimately associated 

 with them to distinguish between them, the so-called " identical " or 

 " homologous " twins. 



Although these two types are both very common, the second rather 

 more than the first, there seems to be in the popular mind no clear dis- 

 tinction between them. That there is a general impression that twins 

 ought to look alike appears from the emphasis placed upon cases where 

 they do not, but that this identity of facial expression does not extend to 

 twins of opposite sex is a fact not commonly apprehended, and instances 

 in literature are not rare in which a young woman in disguise is passed 

 oif as her twin brother, or the reverse. As a matter of fact all twins 

 of opposite sex, as well as many cases in which the sex is the same, 

 belong to the first, or non-identical type, while for the second type an 

 identity of sex as well as of facial expression and other bodily peculiari- 

 ties is a prerequisite. 



Concerning the nature of this peculiarity, the -most plausible and, in 

 fact, the only hypothesis is that twins of the identical or homologous type 

 are produced by the division of a single fertilized egg, while the other 

 type results from the fertilization of two separate eggs, either from the 

 same or different ovaries, and are thus two fundamentally distinct indi- 

 viduals, i. e., a, case of multiple birth such as normally takes place in most 

 species of mammals. As expressed recently by Weismann, 02 (II, p. 54), 

 " Wir haben nun alien Grund, die erste Art von Zwillingen (i. e., fra- 

 ternal) von zwei verschiedenen Eizellen abzuleiten, die letztere Art aber 

 (t. e., duplicate) von e,iner einzigen, welche erst nach der Befruchtimg 

 durch eine Samenzelle sich in zwei Eier getheilt hat.^' ^ Corresponding 



" It seems impossible, witti any degree of certainty, to place the credit for 

 the first enunciation of the above hypothesis. Although often attributed to 

 Francis Galton, Baudouin, gi (p. 274), ascribes it to Camille Dareste, the 

 noted teratologist, who in 1874 defended this theory before the Society 

 d' Anthropologic against the opposition of Paul Broca. Fisher, however, in 

 1866, antedating the statements of either of the above on the subject, advances 

 the same hypothesis to account for the formation of double monsters, stating 

 that they " are invariably the product of a single ovum, with a single vitellus 

 and vitelline membrane, upon which a double cicatricula, or two primitive 

 traces, are developed" (66, P- 208). As Fisher published in a magazine not 

 readily accessible, at least at that time, to foreign investigation (Trans. Med, 

 Soc. State of N. Y.), and as the similarity of separate and united duplicates 

 might not have appealed to them, the formulations of both Galton and Dareste 

 may well have been arrived at independently of Fisher's theory, and the 

 same ideas may have occurred also to others working in the same field, since 

 the hypothesis is of so obvious a nature. 



