THE PROBLEM OF THE FUTURE 23 



common causes — the germ-plasm of the paternal stock or 

 the hereditary factors ; they differ because they are due 

 in part to independent causes. Correlation measures the 

 degree in which any pair of relatives resemble each other. 

 Again, if we take the character in any individual and any 

 phase of his environment, we can by aid of the calculus 

 of correlation determine to what extent that character is 

 associated or independent of the special phase of environ- 

 ment. It is thus that we are now able by a single 

 number lying between o and i to express the intensity of 

 causal relationship between any two variable quantities. 

 The calculus of correlation for the first time enables us 

 adequately to approach such problems as those of nurture 

 and nature, and to determine what weight must be given 

 to these respective factors in our scheme of social reform. 

 Now I propose to draw your attention to three tables. 

 The first gives you a rough idea of relationship between a 

 variety of characters as measured on the scale of correlation. 



TABLE I. SCALE OF CORRELATION OR MEASUREMENT OF 

 INTENSITY OF ASSOCIATION 



Causation or Perfect Correlation . . i-oo 



High Correlation i to -75. 



Right and left femur in man ...... -98 



Middle finger and forearm in man . . . . . -85 



Foot and forearm in man ....... -So 



Middle phalanges of middle and little finger .... -76 



Considerable Correlation -75 to -50. 



Weight and stature in women . . . . . .72 



Middle finger and stature in man ..... -66 



Vaccination and recovery in cases of small-pox . . . -60 



Weight and strength of pull in man . . . . • '55 



Moderate Correlation -50 to -25. 



Light fluctuation and period of variable cluster stars . . -45 



Bank reserve and discount rate ...... -37 



Severity of attack of small-pox and years since vaccination . -32 



High barometer in Portugal and low barometer in Norway . -27 



