mathematically by the tangent of the angle of inclination 

 from the horizontal. 



This coefficient of heredity for stature in man ( =.51) 

 is high, " which indicates either that stature is very largely 

 a matter of heredity or that environmental conditions 

 usually tend to have similar effects on father and son." 

 Studies, show, however, that the coefficient of heredity 

 depends altogether on the nature of the case. Within a 

 "pure line' ' the coefficient is zero; in a mixed population of 

 sweet peas the coefficient is about unity. The coefficient 

 of heredity for speed in horses is not known but would be 

 valuable. 



(d)^ — Value of Biometric Studies 



It is plain that statistical results apply to averages and 

 not to individuals. In heredity biometrics does not take 

 into account the kinds of varieties but the gross total, and 

 for this reason it does not help the geneticist, who desires to 

 know what will definitely occur in any particular case.^ 



Castle says that Gallon's Law works fairly well in 

 cases of blended inheritance, but "as a useful generaliza- 

 tion it is now pretty generally discredited. The reason is 

 fairly obvious. It was an attempt to unify in classification 

 things of unlike character viz,, blending and Mendelian 

 inheritance." 



Biometrics has nevertheless proved useful in showing 

 the falsity of many current beliefs. For example, it is a 

 popular belief that in human matings "opposites attract 

 each other." Pearson shows clearly that there is a de- 

 cided tendency for "like to mate with like." He has found, 

 moreover, that "tall women procreate faster than small 

 women, also that dark-eyed people are more fertile than 

 light-eyed. 



Chapter 8— UNIT CHARACTERS- 



As a result of the investigations of Mendel, DeVries 

 and others, there has arisen the conception of unit charac- 

 ters, which enters largely into recent studies of heredity. 



(1) — A geneticist has made the following facetious comparison between 

 statistical and Mendelian methods of studying. heredity. It is 

 implied in the answer to the question: Why do white sheep eat 

 more than black sheep? 



(2) — Herbert Spencer has the honor of first putting forward the idea 

 of organic units in connection with a theorj' of heredity. In the 

 "Principles of Biology" (1S64) he called these "physiological 

 units." They circulate through the body and "in course of time 

 visit all parts of the organism." Moreover, he assumed that the 

 germ-cells are derived from the parent-body and that modifi- 

 cations impressed upon the body produce similar modifications 

 on the offspring. 



49 



