192 HEREDITY AND EUGENICS 



which appear to be of the " identical " type, except 

 in one or two, or a few characters in which the 

 parents were heteroz3^gous and the germ cells con- 

 cerned happened to carry the corresponding allelo- 

 morphic characters. 



We are so accustomed to seeing diversity in the 

 offspring from a single pair of human parents that 

 we forget what always happens in crosses of homo- 

 zygous types. Any plant breeder will have made 

 numerous crosses in which the F^ offspring are so 

 absolutely alike as to be indistinguishable by any 

 visible differences, or probabh^ even any constant 

 measurable difference. There is no doubt that, for 

 similar reasons, what appear to be identical or 

 monozygotic twins occasionally appear in human 

 families when they are really dizygotic or ordinary 

 twins of the same sex. This clearly helps to account 

 for the occurrence of intermediates between identical 

 and fraternal twins — i.e., twins of the same sex which 

 differ only in a very few characters. 



Windle (1892) has collected numerous cases of 

 identical malformations in twdns, many of which 

 were known to be of the identical type, although, 

 unfortunately, the sex is specifically mentioned in only 

 one or two. A case is described of twins enclosed in 

 the same membranes, in both of which there was a 

 deficiency of the anterior abdominal wall, the in- 

 testines and part of the liver being contained in a thin 

 membranous sac. Another sac, the size of a walnut, 

 hung posteriorly from between the thighs. The 

 latter was probably due to persistence of the neuren- 

 teric canal, dilated by the rachidian fluid to form a 

 sac. Instances in which both twins have the same 

 malformation are said to be not uncommon. But in 

 Rotunda Hospital, Dublin, in the years 1 847-1 854, 

 there were 13,748 women delivered, of whom 233 

 had twins ( = 1 in 59), none, however, showing similar 



