326 STATISTICAL STUDY OF INHERITANCE 



be natural selection combined with heredity, then the single 

 statement which embraces the whole field of heredity must 

 prove almost as epoch-making to the biologist as the law of 

 gravitation to the astronomer." 



Criticisms. — Since the importance of the law is so great, we 

 must devote some attention to certain criticisms which have been 

 made. It goes without saying that those who wish to criticise 

 the basis on which the generalisation is founded must first con- 

 sult the original documents, referred to in the bibliography. 



It must be borne in mind that the Law of Ancestral Inheritance 

 is a statistical conclusion dealing with what is true on an average 

 for a large number of cases. To say that we know of particular 

 cases where it certainly does not hold — where, for instance, the 

 amount of resemblance between an individual and his paternal 

 grandfather is far greater than is represented by the fraction -^V 

 — is no argument against the induction. It is like saying that 

 the statistics showing the percentage of deaths in cases of scarlet 

 fever must be wrong because we know of large families which 

 were visited by the disease without a single fatal result ! 



It may be urged against the crispness of Galton's Law, (i) that 

 the hereditary relation is a complex affair ; (2) that most or- 

 ganic qualities, and the amounts of resemblance in successive 

 generations, can seldom be measured with the accuracy possible 

 in the case of a quality like stature ; and (3) that the actual 

 quota of any character which forms part of a heritage is some- 

 thing different from the expression which that quota finds in 

 development — for the expression depends in part on the con- 

 ditions of nurture. For these and similar reasons it may seem 

 suspicious that the fractions indicating the average contributions 

 of parents, grandparents, great-grandparents, etc., should be 

 representable in such a simple series as ^ -I- ^ + i + . . . • 



The general answer is, of course, that when the data are 

 large enough, the irregularities of result due to particular pecu- 

 liarities, such as a highly prepotent great-grandfather or a very 



