Xo. 562] THE PROBLEM OF INBREEDING 585 



5. Since the only possible infallible criterion of rela- 

 tionship between individuals is common ancestry in some 

 earlier generation, we are led to the practical rule, in 

 measuring the degree of inbreeding in a pedigree, to re- 

 gard all different individuals as entirely unrelated until 

 the contrary is proved by the finding of a common ances- 

 tor. This no doubt appears at this stage of the discus- 

 sion as an exceedingly obvious truism. The reader is 

 urged to accept it as such, and hold fast to it, because it 

 will help him over some apparent paradoxes later. 



The method of calculating coefficients of inbreeding, 

 and their real significance will be made much clearer by 

 the consideration of illustrative examples of their appli- 

 cation. To these we may therefore turn. 



The Calculation of Coefficients of Inbreeding 

 We may first consider some simple hypothetical pedi- 

 grees, before attacking the more complicated ones actu- 

 ally realized in stock-breeding. 

 Illustration I. Continued Brother X Sister Breeding 

 Let us begin with the most extreme type of inbreeding 

 possible, namely the mating of brother with sister for a 

 series of generations. Pedigree Table III gives the pedi- 

 gree of an individual so bred. 



Pedigree Table III. {Hypothetical) 



