CHAPTER I 

 VARIOUS KINDS OF HUMAN TWINS 



Human twins have always been objects of especial 

 interest, partly perhaps on account of the fine humor 

 of the situation, and partly because of the frequent 

 occurrence of ''look-alike" twins or "duplicate" twins. 

 Popular interest has quite naturally focused upon the 

 striking similarity, amounting in some cases to almost 

 complete identity, which exists in certain types of twins; 

 this emphasis upon resemblance is justified by the bio- 

 logical analysis of twinning, as is shown in what follows. 



Biologists have for some time recognized at least two 

 distinct types of human twins: fraternal and duplicate. 

 Fraternal twins may or may not be same-sexed, are 

 usually no more alike than are brothers and sisters, and 

 are believed to be dizygotic, derived from two fertihzed 

 eggs. Duplicate or identical twins are always of the 

 same sex, are almost identical, and are believed to be 

 monozygotic, derived from a single fertilized egg. 



No twins occur in nature at all comparable with the 

 old favorite type which has done such signal service in 

 the drama and in fiction (not to mention the ''movies"), 

 wherein brother and sister are identical. Science, 

 however, can afford to be magnanimous, so let us as a 

 concession to art include in our list these "literary 

 twins. " 



Another type of human twins, stranger than fiction, 

 is found in conjoined twins and double monsters, of 



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