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THE AMERICAN NATURALIST [Vol. XL VII 



of the composition of the generation to which the "com- 

 mon ancestors" of an inbred pair belong. This can be 

 most clearly shown by comparison of two hypothetical 

 pedigrees. In these pedigrees letters will be used to 

 designate animals. 



Pedigree Table I. (Hypothetical) 



Alpha 



Pedigree Table II. (H 1/jiotln tical) 



Xmv it is plain that in both of these pedigrees the num- 

 ber of "free generations" between the mating of the 

 parents of Alpha and Omega respectively (generation 

 number 1) and this common ancestor — o in one case, and 

 o, and p 1 in the other case — is the same, namely 2. Yet 

 every one would agree that the inbreeding involved in the 

 breeding of Omega is much more intense than that in- 



