CHAPTER XIV 



CORRELATION AND THE APPLICATION OF STATISTICS TO 

 THE PROBLEMS OF HEREDITY 



"It is full of interest of its own. It familiarises us with the measurement of variability, and 

 with curious laws of chance that apply to a vast diversity of social subjects. This part of the 

 inquiry may be said to run along a road on a high level, that affords wide views in unexpected 

 directions, and from which easy descents may be made to totally different goals to those we have 

 now to reach. I have a great subject to write upon, but feel keenly my literary incapacity to 

 make it easily intelligible without sacrificing accuracy and thoroughness." 



Natural Inheritance, p. 3. 



A. Introductory. Thus wrote Francis Galton in 1889 when the signifi- 

 cance of correlation and its measurement had impressed themselves upon 

 him. Up to 1889 men of science had thought only in terms of causation, in 

 future they were to admit another working category, that of correlation, and 

 thus open to quantitative analysis wide fields of medical, psychological and 

 sociological research. Turning to the writings of Turgot and Condorcet, who 

 felt convinced that mathematics were applicable to social phenomena*, we 

 realise to-day how little progress in that direction was possible because they 

 lacked the key — correlation — to the treasure chamber. Condorcet often and 

 Laplacef occasionally failed because this idea of correlation was not in their 

 minds. Much of Quetelet's work and of that of the earlier (and many of the 

 modern) anthropologists is sterile for like reasons. 



Galton turning over two different problems in his mind reached the con- 

 ception of correlation : A is not the sole cause of B, but it contributes to the 

 production of B; there may be other, many or few, causes at work, some of 

 which we do not know and may never know. Are we then to exclude from 

 mathematical analysis all such cases of incomplete causation? Galton's 

 answer was: "No, we must endeavour to find a quantitative measure of this 

 degree of partial causation." This measure of partial causation was the germ 

 of the broad category — that of correlation, which was to replace not only in 

 the minds of many of us the old category of causation, but deeply to influ- 

 ence our outlook on the universe. The conception of causation — unlimitedly 

 profitable to the physicist — began to crumble to pieces. In no case was B 



* "Un grand homme [Turgot], dont je regretterai tousjours les lecons, les exeinples, & sur-tout 

 l'amitie, ^toit persuade que les verites des Sciences morales & politiques, sont susceptibles de la 

 meme certitude que celles qui forment le systeme des Sciences physiques, & meme que les branches 

 de ces Sciences qui, comme l'Astronomie, paroissent approcher de la certitude mathematique." 

 Discours preliminaire, Essai sur I'application de I 'analyse a la Probabilite des Decisions, p. i, 

 Paris, 1785. 



t See for example Laplace's memoir in Memoires de VAcademie des Sciences for 1783, pp. 693- 

 702, where J entirely overlooks the correlation between the size of the population and the 

 number of births in evaluating what is really the probable error of the birth-rate. 



pgiii 1 



