1910.| ina Mendelian Population Mating at Random. 39 
should expect it to be larger, for brothers have two parents in common, 
while between father and son comes in the influence of the mother, exerted 
only on the latter member of the pair. Thus Sir Francis Galton, in his 
‘Natural Inheritance’ (p. 133), makes the fraternal resemblance twice as 
great as the paternal. Pearson*, dealing with three characters in 12 tables 
of over 1000 each, found the average correlation for parent and child to 
be 0°46, and for three characters in nine tables the average fraternal 
correlation to be 0°52. For eye-colour in manft he found the parental 
correlation 0'495 and the fraternal 0-475, showing a slightly reversed 
relationship. Both, however, compel us to affirm that the degrees of 
resemblance between parent and offspring and between brethren are very 
nearly equal and differ but slightly from 0°5. The values which Mendelian 
theory leads us to expect are shown in Table IX. 
(4) Further conclusions can be drawn now that the measurement of 
resemblance between pairs of cousins and -between uncles or aunts and 
nephews or nieces has been completed. Judged by a great variety of 
characters, the cousin relationship has been found to be as close as, if not 
closer than, the avuncular relationship. This result is, of course, no more 
remarkable than that between the parental and fraternal resemblances 
referred to above. However, in view of the publication of a large number 
of avuncular correlations which have been worked out in the Statistical 
Laboratory at University College, it seems desirable to show that it is at 
least consistent with some one or other theory of heredity. From Table IX 
it will be seen that on the Mendelian theory the relationship between 
cousins is as close as that between uncle and nephew. 
The point is one which is of considerable importance in medical diagnosis. 
The parents’ brothers and sisters have been usually included, and the cousins 
excluded, in considering the family history. The fact, however, that a man’s 
cousins resemble him as closely as do his uncles and aunts shows that the 
cousins, usually more numerous, provide at least as good material for an 
estimate of his tendencies as his parents’ brothers and sisters. 
The paradox that a man’s father’s brother’s son resembles him, on the 
average, as closely as his father’s brother, although the additional ‘influence 
of the father’s brother’s wife has come in, is only a paradox so long as we 
overlook the fact that somatic and gametic characters are not causally 
related but only correlated. Almost any determinantal theory of heredity 
brings out the fact that, on the average, a man resembles his father as 
closely as he does his brother, and, further, that he is not more like to his 
* * Biometrika,’ vol. 2, pp. 378, 387. 
t ‘ Phil. Trans.,’ A, 1900, vol. 195, p. 106. 
