HEREDITY IN MAN 105 



figure, one half or -5 is termed the coefficient of correlation 

 between father and son, and is in this case a measure of 

 the intensity of inheritance. The method of arriving at 

 the exact figure is much more elaborate than the above 

 condensed table would seem to imply, and is a general 

 method of measuring the dependence of one thing upon 

 another. It is a figure which, when the dependence is 

 complete, gives the figure i ; when there is complete 

 independence, the figure o ; and when the dependence is 

 partial, some figure in between. Thus, in the case in 

 question, if the tallest fathers always had the tallest 

 sons, the next tallest fathers the next tallest sons, and so 

 on, the correlation would be i--i.e. inheritance would be 

 complete. If, on the other hand, there were no inheritance, 

 and tall fathers just as often had short sons as tall ones, 

 then the correlation would be o. 



It is quite essential to grasp clearly this idea of corre- 

 lation, the measure of dependence of one thing upon 

 another, as it is impossible to understand otherwise the 

 bulk of the work done upon inheritance in man. 



With this knowledge of the inheritance of a typical 

 physical characteristic which can serve as a standard of 

 comparison, we can consider the inheritance of a variety 

 of different characteristics, and we will begin with 

 Insanity, a condition which as everyone knows is liable 

 to run in families. One of the best investigations on this 

 subject was that made by Heron, who used as a basis 

 the archives of the James Murray Royal Asylum, Perth, 

 which contains the records of 331 family trees. Heron 



lation, but through the mediation of the regression coefficient. It 

 does not, however, appear necessary to burden the untechnical 

 reader with this refinement. 



