CHAPTER VI 



THE PURE LINE AND SELECTION 

 1. GALTON'S LAW OF REGRESSION 



GALTON was one of the first * to attempt to express 

 mathematically the relationship between parents and 

 offspring by means_ojF__ir^atmg_^tatisjically a single 

 unit character. According to Galton, a mathematical 

 expression of the relationship between two generations 

 should serve as a corner-stone of heredity. 



What Galton did was to take human stature as a 

 unit character in comparing 204 English parents and 

 their 928 adult offspring, because human stature is 

 not complicated by environmental influences and is 

 consequently, a purely hereditary matter. 



The results of his measurements expressed in inches 

 are shown in Figure 23 in which the circles connected by 

 the diagonal line represent the graded parental heights, 

 while the arrowpoints indicate the average heights of 

 the offspring in each group. 



This illustrates Galton's Law of Regression or the 

 tendency in successive generations toward mediocrity. 

 The law may be stated as follows : 



Average parents tend to produce average children; 

 minus parents tend to produce minus children; plus 



1 "Hereditary Genius," 1869. 



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