HUMAN EYE COLOUR 41 



The character which I propose to consider now 

 resembles the case of stature in peas, in that the 

 hybrid does not differ externally, so far as we 

 can see, from the pure dominant. It is also of 

 especial interest because it is one of the few 

 Mendelian characters which have been discovered 

 in man. 



When we speak of the colour of the human eye, 

 we are speaking only of the colour of a particular 

 part of it namely the iris, which is the only part 

 of the eye the colour of which varies to any great 

 extent. There is, of course, an almost infinite variety 

 in the colour of the eye ; but the various types of 

 colour can be grouped into two large classes according 

 as to whether there is brown pigment on the outer 

 surface of the iris or not. When the whole of this 

 surface, which is the one that we see when we look 

 at the eye, is covered with brown pigment, the eye 

 is brown ; and when there is no such brown pigment on 

 the outer surface of the iris the eye is generally blue. 

 The word duplex has been coined to denote those 

 eyes in which there is some brow^n pigment in front 

 of the iris ; and simplex to denote those eyes in 

 which there is none.* Speaking generally, duplex 

 eyes are brown, and simplex are blue. But as the 

 two classes, duplex and simplex, include the whole 

 range of human eye colour it is evident that duplex 

 does not simply mean brown and simplex blue, 



* These terms were introduced by Mr. C. C. Hurst, who was the first to 

 discover, investigate and record the Mendelian inheritance of eye colour 

 in man. 



