viii] HEREDITY 107 



illustrated by the inheritance of pigmentation in 

 man, in skin- and hair-colour. In the case of hair- 

 colour, Hurst has given evidence that bright red 

 behaves as a recessive to dark-coloured hair, and 

 that to some extent at least segregation takes place. 

 But the shades of hair-colour graduate into one 

 another so continuously that it is impossible to 

 place them with confidence in Mendelian categories, 

 and further the colour alters so greatly between 

 infancy and maturity in many persons that classifi- 

 cation is difficult. Many children for example have 

 bright red hair, in whom during adolescence the 

 colour deepens to brown, while other members of 

 the same family, whose hair has hardly differed from 

 the first during childhood, keep the bright red until 

 middle life. In families with red hair we may see 

 clear evidence of segregation between red and dark 

 brown hair-colour, but the differences between the 

 originally red-haired individuals show that some 

 contain a darkening factor which is absent in the 

 others. Probably then in human hair-colour there 

 are a number of factors which interact upon one 

 another in a way even more complex than in the 

 hair-colour of mice mentioned in a previous chapter ; 

 and where rigid experiment is impossible, the analysis 

 of these factors is almost hopeless. The same remarks 

 perhaps apply to the various eye-colours within the 

 class with pigmented iris, and the very frequent but 



