CHAPTER XXXVII 

 TWINS AND HEREDITY 



I. THE BIOLOGY OF TWINS 



Twins have always been a source of interest and amusement to 

 other people. There is something inherently comic in the existence 

 of two individuals so similar that their identities are likely to be con- 

 fused. Stories and plays have been written, the chief plot of which 

 involves the mistaken identity of two brothers, two sisters, or a brother 

 and sister. Quite recently I came to know an attractive lady who told 

 me an interesting story of her girlhood — a story from real life that bears 

 out those of drama and fiction. This young woman had a sister so 

 much like herself that her best friends could not tell them apart. It so 

 happened that a young man became attracted to one of the twins and 

 called practically every night to pay his court. This assiduous wooing 

 soon became burdensome, and, to relieve herself, the young lady had 

 her twin sister substitute for her on some of the evenings. In the 

 course of time the young man approached a proposal of marriage, but 

 unfortunately selected an evening for popping the question when the 

 substitute twin was on duty. He told her that, though some claimed 

 to see no difference between her and her sister, he himself thought 

 there was a great difference and all in her favor. He said he could not 

 see why other people thought them so much alike. The substitute 

 was forced to confess the deception; but in spite of this, the young man 

 insisted that she was the one he wanted, and suspected her of merely 

 testing his fidelity. In the resultant confusion the young man made 

 such an ass of himself that neither twin had any further use for him. 



This is, however, not to be a romance about twins, but a serious 

 discussion about the scientific value of twins; so let us get on with it. 



Various kinds of twins. — Several different kinds of human twins 

 are recognized, among which the most significant are identical (dupli- 

 cate) twins, "Siamese twins," and fraternal twins. Duplicate twins 

 are, as the name implies, extremely similar in their personal charac- 

 teristics (Fig. 91). They are always of the same sex in a pair, have 

 the same or very similar temperaments, dispositions, and mental ca- 

 pacities. They are believed to be derived from the division of a single 

 zygote. There is no such reality in nature as that useful and romantic 



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