Correlation and Application of Statistics to Problems of Heredity 37 



Constitution and Ancestry of Hazel- Eyed Sibships. 



the general population percentage. The distribution of the grandparents of 

 a hazel-eyed person is practically the same as that of the general popula- 

 tion. From these data Galton concludes as follows : 



"The total result in passing from Generation III to I, is that the percentage of the light eyes 

 is diminished from 60 or 61 to 45, therefore by one quarter of its original amount, and that the 

 percentage of the dark eyes is diminished from 26 or 27 to 23, that is to about [1 by about] one- 

 eighth of its original amount, the hazel element in either case absorbing the difference. It follows 

 that the chance of a light-eyed parent having hazel offspring is about twice as great as that of 

 a dark-eyed parent. Consequently since hazel is twice as likely to be met with in any given 

 light-eyed family as in a given dark-eyed one, we may look upon two-thirds of the hazel eyes as 

 being fundamentally light and one-third of them as fundamentally dark. I shall allot them 

 rateably in that proportion between light and dark and so get rid of them. M. Alphonse de 

 Candolle has also shown from his data that yeux gris (which I take to be equivalent to my hazel) 

 are referable to a light ancestry rather than to a dark one, but his data are numerically insuffi- 

 cient to warrant a precise estimate of the relative frequency of their derivation from each of 

 these two sources." (pp. 407-8.) 



I find it very difficult to follow this reasoning, or to see from the table 

 above its validity. It would seem to be essential to follow up the particular 

 ancestry of each hazel-eyed individual, before we can draw the conclusions 

 that Galton does from the massed numbers of children, parents and grand- 

 parents. Galton and de Candolle at least admit the difficulty of the hazel 

 eyes; many Mendelian writers speak only of "brown" and "blue" eyes; 

 others speak of hazel-eyed persons as heterozygotes*. 



Galton having thus disposed of his yeux gris, now turns to the same 

 multiple regression formula as he has used for stature, namely he makes 

 the regression coefficient \ for a parent, ^ for a grandparent and so on to 

 higher ancestry. He also makes use of what is, I believe, an erroneous hypo- 

 thesis, at any rate one inconsistent with his multiple regression coefficients, 



* Sometimes a definition is given of pure blue eyes as being those without anterior pigment. 

 According to one ardent Mendelian this can always and only be tested with a lens; another 

 accepted relatives' statements, and came to the same conclusion without a lens. From twelve cases 

 in which both eyes were carefully examined with a lens and thus found to be without anterior 

 pigment, the excised eye when sectioned and examined microscopically showed quite clearly 

 anterior pigment. Hitherto I have failed to come across any eye, however blue, which is without 

 some anterior pigment when sectioned. At what degree of pigmentation does the recessive 

 character cease? 



