36 THE RELATIVE STRENGTH OF 



food, bad air, or parental neglect may account for the presence of any 

 disease. There is always the diathesis as a contributory source, and 

 the fact that all the children in the same family may not be attacked, 

 shows that the variability within the family also plays its part. 



Again my critics will, I hope, pardon me if I say that I am not 

 convinced when I am asked whether this or that " is not a fact." For 

 example, that it is a fact that in the better homes the children are 

 more studious, or that the more respectable parents keep their 

 children at home. Either may be the truth, but what we want are 

 actual numerical measures of the effect of these supposed causes in 

 invalidating the apparently small influence of the environmental as 

 compared with the hereditary factor. 



What we want are more data before we conclude that results are 

 paradoxes because of such or such an explanation being "a fact" or 

 "a matter of common experience." 



Let us attempt to get any information we can on the problem of 

 whether the better class parents keep their children at home, and so 

 their offspring are more studious and suffer more from myopia ^ 

 Now, I know of no statistics which at all touch this point of the 

 studious character of the offspring of better class parents, but I 

 suppose the argument to be that the children are kept more at close 

 work and that this damages their sight. Now, it would be possible 

 to compare the sight of children of the same age who came to school 

 at different ages originally, and thus find out whether those who began 

 to read earlier have markedly worse sight. It would be very desirable 

 to collect statistics de ?iovo on this point. The only material that 

 I know of bearing on the subject is that used by Miss Harrington and 

 myself, and provided by Cohn, who gives the correlation tables of 

 sight and years of school life, and sight and age. We have shown 

 that the relationship is more intimate between sight and age than 

 between sight and years of school life, and that, considering the high 

 correlation of years of school life and years of age, the latter is most 

 probably only a derived result of the former. Now, we have recently 

 re-worked Cohn's data and discovered the correlation between the 

 ages at which the child began to read and the degree of myopia 

 at a constant age. The partial correlation co-efficient is : 



0'04, if age and length of school life have a correlation of o'8 

 o"i3, if age and length of school life have a correlation of o'p. 



^ The reader of our memoir will remember that our conclusion was, not 

 that the better homes produced more myopia, but that there was no tnarked 

 relationship between bad homes and defective sight. 



