VARIATION AND HEREDITY 217 



vidual. Thus there is a racial inheritance, which 

 is the common possession of all the individuals of 

 any group or race. When we try to picture the 

 characteristic features of this sort of heritage, we 

 find that we must abstract the qualities of a great 

 number of individuals and make a sort of composite 

 or average of them, i.e. such a description is that 

 of an ideal individual instead of a real one. We 

 unconsciously do this whenever we form a mental 

 picture of an organism, such as a trout, or an apple 

 tree, or a butterfly. 



Gallon's Law of Ancestral Inheritance. We have 

 found that one fruitful way of comparing the like- 

 ness or difference between two groups of individuals 

 is by calculating the abstract index of correlation 

 of the variations. Correlating the resemblances be- 

 tween parents and offspring gives us an index of the 

 degree of inheritance which we may assume the latter 

 derive from the former (on the average, not indi- 

 vidually). Sir Francis Galton, a famous student 

 of heredity, calculated in this way the degree of 

 inheritance received by the offspring from parents 

 and from more remote ancestors, and concluded 

 that, on the average, the individual receives one half 

 his heritage from his two parents, one fourth from 

 his four grandparents, one eighth from his great- 

 grandparents, one sixteenth from the great-great- 

 grandparents, and so on. Subsequent calculations 

 have modified these fractions, increasing the per- 

 centage derived from the immediate parents and 



