CHAPTER VI 



HEREDITY OF COLOUR— continued. 



Eye-Colo?crs. Variations in Colour of the Iris — Deficiency 

 of Eye-Pigments in some Coloicred Types. 



The colours of eyes present some special genetic pro- 

 blems which are worth examining in detail. 



Pigments are formed in several parts of the vertebrate 

 eye, but chiefly in the choroid and the iris. In all normal 

 eyes the choroid contains much deep black pigment, and to 

 this fact the blackness of the pupil is of course due. The 

 common differences in eye-colour with which we are familiar 

 in man are not caused by changes in the colour of the pupil 

 but by variations in the structure and pigmentation of the 

 iris alone. Such differences, being conspicuous factors in 

 individual and racial differentiation, have long attracted the 

 attention of anthropologists. With the object of elucidating 

 the heredity of eye-colours extensive pedigrees were 

 collected by Galton, and biometrical studies of a similar 

 kind have been published by Professor Pearson and his 

 assistants. Various conclusions of a statistical nature have 

 been based on these data, but as no critical analysis was 

 attempted, such results are devoid of genetic importance. 

 As we now know, moreover, the method of classification 

 employed in collection was unfortunately defective. The 

 categories were not sufficiently precise, and the material 

 would give no reliable indications if analysis were attempted. 



As the result of comprehensive investigation carried 

 out in a single district on Mendelian lines, Hurst has 

 succeeded in making an important advance*. Recognizmg 

 that such descriptions as "light," "dark," and so forth are 



* Similar evidence has also been published by Davenport (107). 



