VARIOUS KINDS OF HUMAN TWINS 9 



which the Siamese twins furnish a stock example. There 

 appears to be a graded series of types ranging between 

 duplicate twins and the less monstrous types of con\ 

 joined twins; similarly, the conjoined twins appear to 

 grade into the various kinds of double monsters. Some 

 conjoined twins are very lightly connected and some 

 double monsters are so closely united that one individ- 

 ual may be a mere degenerate parasite upon the other. 

 The fact that very lightly conjoined twins exist would 

 point to the probabihty that such twins might some- 

 times be born separately. This is Wilder's view of the 

 relaition between duplicate twins and double monsters. 

 Lightly conjoined twins are always of the same sex and 

 are strikingly similar; there seems to be no question 

 as to their monozygotic origin. What more natura.1, 

 therefore, than to infer that separate twins which are 

 of the same sex and strikingly aUke are also monozygotic ? 

 Considerable direct and indirect evidence that 

 monozygotic twins are of frequent occurrence in man 

 is available. Perhaps the most conclusive evidence foT 

 this idea is to be found in a study of the sex-ratios 

 of twins. Data are available from several different 

 sources, but perhaps the best is that given by Nichols,' 

 which is herewith presented: 



' J. B. Nichols, Memoirs of the American Anthropological Associa- 

 tion, I (1907). 



