IIL.—ASSOCIATION. 23 
If, on the other hand, it is desired to exhibit the conditions 
amongst the imbecile, the second will be preferable. 
Proportion of deaf-mutes amongst 0 
the imbecile (4B)/(4) . _ . }9 2 Dep shousanl 
Proportion of deaf-mutes in the } 05 
whole population (B)/N . . 
Either comparison exhibits very clearly the high degree of asso- 
ciation between the attributes. It may be pointed out, however, 
that census data as to such infirmities are very untrustworthy. 
Example vii.—Eye-colour of father and son (material due 
to Sir Francis Galton, as given by Professor Karl Pearson, Phil. 
Trams, A, vol. cxcv. (1900), p. 138; the classes 1, 2, and 3 of the 
memoir treated as light). 
Fathers with light eyes and sons with light eyes (4B) . 471 
2 » 2 not light » (4B) . 151 
»» not light " light y»' {aB) 8 
» ” “ not light ,, (af) 230 
Required to find whether the colour of the son’s eyes is 
associated with that of the father’s. In cases of this kind the 
father is reckoned once for each son; e.g. a family in which the 
father was light-eyed, two sons light-eyed and one not, would be 
reckoned as giving two to the class 48 and one to the class 4 B. 
The best comparison here is— 
Percentage of light-eyed amongst the sons 
of light-eyed fathers . : ‘ ) 76 per cent. 
Percentage of light-eyed amongst the sons \ 39 
of not-light-eyed fathers . : ¥, 2 
But the following is equally valid— 
Percentage of light-eyed amongst the 
fathers of light-eyed sons . id k } 76 per cent. 
Percentage of light-eyed amongst the 40 
fathers of not-light-eyed sons : : » 
The reason why the former comparison is preferable is, that we 
usually wish to estimate the character of offspring from that of 
the parents, and define heredity in terms of the resemblance of 
offspring to parents. We do not, as a rule, want to make use of 
the power of estimating the character of parents from that of their 
offspring, nor do we define heredity in terms of the resemblance 
of parents to offspring. Both modes of statement, however, 
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