K. Pearson 
139 
that line had a slope of 1 in 1, the brother of 74-5 would have a mean brother 
of 74-5 cephalic index. If it had no slope at all the brother of 74-5 would have 
a brother like the mean of the general population. In the one case we have 
absolute resemblance, in the other case no resemblance at all. The actual 
degree of resemblance, our brothers being equally variable, is measured by the 
steepness of this regression line. In our case that steepness is "49, almost "5 or 
1 in 2. That is the measure of fraternal resemblance in brothers for cephalic 
index — the correlation between brothers as we term it. 
Now we have learnt two great features of inheritance in man. First, that 
the points in Diagram I., within the limits of observation, are on a line, and 
secondly, that the slope of this line is about "5. Are these results true for 
characters other than the cephalic index ? Undoubtedly for all the physical 
characters yet w^orked out in man. Here are additional illustrations: see Diagrams 
II.- — ^IV.* We cannot hesitate about the regression line being essentially linear. 
Has it for brethren usually a slope of about '5 ? 
Diagram IV. Resemblance of Mother and Daughter in Span. 
63 53 64 65 56 57 58 59 60 61 62 63 64 65 66 67 68 60 70 71 72 
Blather's Span in inches. 
In Table I. are given my observations on some 1000 families for adult brothers 
and sisters. You will see that the steepness of the regression line is essentially 
about "5. 
In Table II. are given my observations on the head measurements of school 
children. We note at once precisely the same convenient number '5. 
* Diagrams 11. and IV. are reproduced from the memoir by the author on "Inheritance of the Physical 
Characters in Man," Biometrika, Vol. ii. pp. 362-3, and Diagram III. from an article in the same 
journal. Vol. ii. p. 216, on the "Law of Ancestral Heredity." 
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