EVOLUTION 



491 



Thus we see that exclusive inheritance tends to in- 

 crease the desfree of resemblance in the direct line verv 

 considerably, and also in the avuncular line, while for fair- 

 sized families it would leave the average degree of resem- 

 blance practically unaltered for brethren. 



Now Mr. Galton has pointed out ^ that eye-colour in 

 man rarely, if ever, blends : " If one parent has a light 

 eye-colour and the other a dark eye-colour, some of the 

 children will, as a rule, be light and the rest dark ; they 

 will seldom be medium eye-coloured like the children of 

 medium eye-coloured parents." 



Thus eye-colour seems a very suitable character upon 

 which to investigate exclusive inheritance, and as Mr. 

 Francis Galton kindly allowed me free use of his data, I 

 was enabled to test the above numbers. In each case I 

 had some 800 to i 500 pairs of relatives of each grade to 

 deal with, and the tabulation, reduction, and calculation of 

 such an amount of material took many weeks of work. 

 I will not give here the values obtained for each of the 

 four parental, the eight grandparental, the eight avuncular, 

 and the three fraternal sorts of relationship, but merely 

 the mean values in each case. 



Intensity of Eye-Colour Inheritance 



Now it will be clear to the reader that the first three 

 results are quite incompatible with the numbers obtained 

 for blended inheritance. They are much closer to those 

 for exclusive inheritance. That obtained for fraternal 

 inheritance is, indeed, not so far removed from the value 

 given by that hypothesis, but just in this relationship the 

 two hypotheses give very similar results. The correlation 



^ NaUiral liilicnlaiice, p. 139. 



