78 HEREDITY 



the inheritance would have been complete. In other 

 words, the degree of inheritance is measured by the 

 size of the angle which the line AB makes with the 

 horizontal. In Fig. 11, the position of this line is just 

 about midway between the horizontal and 45. The 

 figure representing the size of this angle, as a fraction of 

 45, is termed the coefficient of heredity, and in the case 

 discussed it works out at 0*51. This means that the size 

 of the son is on the average about half-way between 

 the size of the father and the mean size of the race. 



The simple method of determining the coefficient 

 of heredity which we have used, assumes that variation 

 is normal, and similar in the parents and the offspring ; 

 also that the line joining the series of points in the 

 figure shows no definite bend. If these conditions are 

 not fulfilled, more complicated methods must be used. 



The coefficient of heredity for stature in man is 

 exceptionally high, which indicates either that stature 

 is very largely a matter of heredity, or that environ- 

 mental conditions usually tend to have similar effects 

 in father and son. The value of the coefficient varies 

 in different cases. It has been calculated, for instance, 

 for the size of litter in pigs, i.e. for the number of young 

 produced at a birth by a sow and one of her female 

 progeny, and the figure obtained was 0'07. This in- 

 dicates that, in the material used, the number of young 

 produced at a birth is largely influenced by causes 

 other than heredity, and that there are only relatively 

 slight inborn tendencies to produce large or small litters. 



Gal ton, who was the pioneer in the statistical study 

 of inheritance, attempted to reduce to one general 

 law the result of his studies of heredity ; that is, he 

 attempted to determine general figures for the average 

 degree of resemblance between parent and offspring, 

 grand-parent and offspring, and so on. Professor 

 Pearson, on the basis of a great deal more experimental 

 work, has stated " Galton's Law " in a modified form. 

 But the fact has emerged that the coefficient of heredity 

 must depend altogether on the nature of the case. 

 Within a pure line, the differences that exist appear to 



