STATISTICAL STUDY OF HEREDITY 77 



stature classes of the fathers. One can then measure 

 vertical distances to represent the mean size of the sons 

 of each group. From Fig. 11, where this has been done, 

 it will be evident that the mean size of the sons of 

 63-inch fathers is about 66-3, of 66-inch fathers about 

 67'6, and so on. The extreme classes are not included, 

 as the number in each is too small to give a reliable 

 result. 



FIG. 11. 



It will be seen that the series of points thus obtained 

 falls roughly in a straight line. That the line is not per- 

 fectly straight is probably due to the limited number of 

 cases included. We can, however, draw in the straight 

 line (AB) which most nearly touches the whole series 

 of points, and which, in the figure, occupies an oblique 

 position. It will be evident that if the line AB were 

 horizontal i.e. if all classes of fathers had had about 

 the same average size of sons the inheritance would 

 have been zero ; if, on the other hand, this line had been 

 inclined at 45 i.e. if each class of fathers had tended 

 to have sons of the same average size as themselves 



