104 HEREDITY [ch. 



a thing which should not only be useful to insurance 

 offices, but to all thinking men, for it may ultimately 

 become the basis for deciding on the propriety of 

 marriage by members of tainted families. In general 

 it appears that the resemblance of a child to its grand- 

 parent is rather more than half of that to its parent, 

 and that the resemblance between uncle and nephew, 

 or between first cousins, is very slightly less than 

 between grandchild and grandparent. 



We may now turn to definitely discontinuous cha- 

 racters in man, some of which are clearly Mendelian 

 in their inheritance. One of the most interesting 

 cases is that of eye-colour. Hurst [17] has shown that 

 complete absence of pigment in the front of the iris 

 is recessive to the presence of pigment ; that is to 

 say, that two pure blue-eyed people have only blue- 

 eyed offspring, but that a blue-eyed individual married 

 to one with any brown or yellow in the iris may have 

 children with pigmented eyes and that two pigmented 

 parents have pigmented children, with or without 

 a proportion of blue-eyed in addition. Within the 

 pigmented class there is great range of variation, 

 from a small yellow rim round the pupil to com- 

 pletely dark eyes, and the relation of the various 

 pigmented types to each other has not yet been 

 analysed. But that the characters ' pigmented ' and 

 ' non-pigmented ' are a Mendelian pair, his evidence 

 leaves no doubt. This is thus a case in which the 



