BIOMETRICS 167 



amount of fertility. Pearson has proved the inheritance 

 of fertility from mother to daughter, from father to son, 

 and from paternal grandmother to granddaughter, in 

 accordance with the Law of Ancestral Inheritance, to 

 which we now turn our attention. 



V.THE LAW OF ANCESTRAL INHERITANCE. 



We have in a previous chapter (IV.) shown the share 

 that parents have in the constitution of their children, 

 and also the contribution made by ancestors to any given 

 individual. Dealing there with the germ-cells as the 

 constituent factors, we discussed the various possibilities 

 of inheritance. We could there only point out what were 

 the possibilities, without at the same time being able to 

 say what occurs in any given instance. Now, the statistical 

 method takes us a little farther. Without telling us what 

 will happen in an individual case, it can at least give us 

 information as to what will, on an average, be the fre- 

 quency or probability of such a case. We may not know 

 who is going to die, or from what cause any given person 

 is going to die in any year, but statistics enable us to tell 

 how many die on an average in any given year, and from 

 what diseases useful knowledge, as far as it goes. In 

 the same way, though we cannot learn how much each 

 individual parent and ancestor contributes towards the 

 constitution of a given organism, the biometrical method 

 enables us to formulate a " Law of Ancestral Inheritance," 

 which tells us how much on the average is the amount of 

 contribution of each parent and each ancestor. 



The contributions of the several progenitors of any given 

 individual have been calculated by Galton on the basis 

 of the Coefficient of regression as follows : " The two 

 parents between them contribute, on the average, one-half 

 of each inherited faculty, each of them contributing one- 

 quarter of it. The four grandparents contribute between 

 them one-quarter, or each of them one-sixteenth, and so 



