48 HEREDITY AND EUGENICS 



present genetic knowledge. It is well known that 

 in rabbits and guinea-pigs factors for coat colour also 

 often affect e^^e colour. 



Having stated some of the facts as found, several 

 criticisms of the present Mendelian position as regards 

 e3^e colour are necessary. In the first place, many 

 grades of brown exist, both as regards shade of 

 colour of the iris and distribution of pigment. In an 

 accurate study of eye colour these shades and varying 

 distributions should be distinguished, and only lumped 

 together for certain statistical purposes. It may 

 turn out that all the shades of iris pigmentation do 

 segregate sharpty from pure blue, but much more 

 extensive and accurate data will be required than are 

 at present available before any certain conclusions 

 can be drawn. It appears that even a difference 

 in pigmentation of the two eyes ma}^ be inherited in 

 certain families, and when the effects of various 

 abnormalities of the eye in distorting or inhibiting 

 the pigmentation of the iris are considered, the neces- 

 sity for accurate and prolonged observation is obvious. 

 On the other hand, in the light of the complications 

 as regards eye pigmentation disclosed in Drosophila, 

 it is by no means improbable that w^hen sufficiently 

 anah'sed, the pigmentation of the iris in man will be 

 found also to follow^ Mendelian laws. But it is neces- 

 sary to emphasise the necessity of very accurate 

 and detailed first-hand observations of parents and 

 children. The existence of all intergrades of colour 

 and distribution of pigment in the iris is well known. 

 Whether the detailed facts will bear a complicated 

 Mendelian analysis remains to be seen, but there is 

 nothing at present to negative that possibilit}^. 



The writer recently had the opportunit}'^ of ex- 

 amining the e3^e colour of people in Bergen, Norway. 

 Only about one in fifteen would be roughly classed 

 as brown-eyed, but the blues varied continuously 



