PROBLEM OF NURTURE AND NATURE 43 



health, intelligence, caution, and many other desirable factors, and 

 these not only in one parent but in brothers, sisters, aunts, uncles and 

 grandparents and treat the cross-correlation of these with the character 

 under discussion. But although every improvement in stock would 

 reflect itself in improvement in offspring, correlations cannot be added 

 together any more than forces by simple arithmetical addition. You 

 do not combine two hereditary correlations any more than two 

 environmental correlations by mere addition. You must proceed by 

 a combination process which is of course familiar enough to the 

 trained statistician. 



Yet here is a statement which the Editor of the Eugenics Review 

 admits without contradiction to its pages 1 : 



"The point that we wish to make is this. In the face of so much 

 ignorance concerning, not only heredity itself, but also its complement, 

 the influence of environment, how can any one be justified in making 

 sweeping generalisations with reference to these subjects? 



Such generalisations, however, are made. It is said that we have 

 a definite proof that inheritance is of far greater strength than 

 environment. This argument takes the following shape. The corre- 

 lations between parent and offspring for a number of features have 

 been calculated, and the mean is found to be somewhere about -5. 

 Correlations between individuals and various aspects of their environ- 

 ment have also been worked out as, for instance, mental ability and 

 conditions of clothing, or between myopia and the age of learning to 

 read 2 and the mean value is found to be about -03. It is then said 

 that the mean 'nature value' is at least five to ten times as great as 

 the mean 'nurture value,' and upon this is founded the generalisation 

 that 'nature' is of far greater importance than 'nurture' 3 . It may 

 be questioned, however, whether such a comparison does not involve 

 a serious mistake. For if we consider the two mean values that are 

 compared, we find that, whereas the 'mean nature value' is the mean 

 value of a number of observations, all of which provide a full measure 

 of the strength of heredity, the ' mean nurture value ' is the mean value 

 of a number of observations, each of which measures only the strength 

 of some one isolated aspect of environment. It would appear then that 

 the full strength of inheritance has been compared, not with the full 

 strength of environment, but with the average of a number of small 



1 Vol. v, p. 219, in an article by A. M. Carr-Saunders. 



2 As the writer phrases this correlation, it is very liable to be misinterpreted. 

 What the Galton Laboratory did was to show that myopia was very markedly 

 inherited, and that the theory that it was largely due to school environment was 

 incorrect, because children who began to read late, i.e. went late to school, were not 

 less myopic than those who went early. 



8 Karl Pearson, Nature and Nurture, Eugenics Laboratory, Lectures VI, p. 25. 



