vi] Eye-Colours 109 



pigment, and many eyes which would be called ''dark" 

 are without it. The differences between the non-pigmented 

 "dark" iris and the non-pigmented "light" iris are caused 

 by structural differences in the iris itself. 



Apart from its scientific importance it will be evident 

 that this discovery is of considerable practical interest as a 

 contribution to human genetics. Much nevertheless remains 

 to be done in the application of precise methods to the 

 problem of human eye-colours. For example, amongst the 

 dominants examined by him Hurst recognized two distinct 

 types: (i) self-coloured irides in which the pigment is distri- 

 buted over the whole surface of the iris, (2) ringed irides 

 in which the pigment was absent from the periphery but 

 existed as a ring round the pupil. The relations between 

 these two types could not be satisfactorily determined. All 

 that the evidence showed was that the "self" behaves as a 

 dominant to the "ringed." But on analogy with other such 

 cases we of course should expect that these distinctions 

 were caused by distinct determining factors, either of which 

 might be borne by the non-pigmented recessive without 

 revealing its presence. If this were so, some of the non- 

 pigmented should, on breeding with the ringed, have off- 

 spring with self-coloured irides, just as some albino rabbits 

 on breeding with Dutch-marked rabbits may have self- 

 coloured offspring. Curiously enough, however, in an 

 ample series of observations, Hurst met with no such 

 occurrence, though examples of self parents producing the 

 three kinds of offspring, self, ringed, and non-pigmented, 

 were not rare. Here, at present, the matter rests *. 



Hurst's observations relate entirely to the population 

 of a small English village where naturally only a limited 

 range of types would be met with. The inter-relations of 

 the eye-colours of strongly marked racial types doubtless 

 will provide an excellent field for the application of a similar 

 analysis. It is likely too that such work would result in the 

 discovery of the principles governing the association of 

 certain eye-colours with particular colours of the hair, and 



* Hurst has suggested with some plausibility that the absence of any 

 self-coloured irides among the offspring of ringed x non-pigmented may be 

 attributed to the fact that the offspring examined were mostly school- 

 children. He thinks it not improbable that some of the ringed may 

 subsequently develop into self-coloured irides. 



