FACTS OP INHERITANCE 143 



lent to two ordinary members of a good stock, 

 but " tbe children of the former will tend to re- 

 gress ; those of the latter will not " (p. 198). 



Let us give a concrete illustration from Prof. Karl 

 Pearson's " Grammar of Science " (1900, p. 454) : 

 " Fathers of a given height have not sons all of a 

 given height, but an array of sons of a mean height 

 different from that of the father and nearer to the 

 mean height of sons in general. Thus, take fathers 

 of stature 72 inches, the mean height of their sons 

 is 70'8 in., or we have a regression towards the mean 

 of the general population. On the other hand, 

 fathers with a mean height of 66 in. give a 

 group of sons of mean height 68"3 in., or they have 

 progresse'd towards the mean of the general popula- 

 tion of sons. The father with a great excess of the 

 character contributes sons with an excess, but a 

 less excess of it ; the father with a great defect 

 of the character contributes sons with a defect, 

 but less defect of it. The general result is a sensible 

 stabihty of type and variation from generation to 

 generation." 



There seems no reason to doubt the general 

 occurrence of regression towards mediocrity, though 

 the law requires modification in regard to charac- 

 ters which are subject to keen selection, either 

 natural or sexual, and though it does not apply to 

 sharply defined "unit characters" which do not 

 blend. 



Galton's Law op Ancestral Inheritance. — 

 It is necessary, however, to ask what this statis- 

 tically estabhshed fact of fiUal regression really 

 means biologically. 



In all ordinary cases of reproduction the off- 

 spring has a strictly dual or bi-parental inheritance. 



