Phenomena of Inheritance 



77 



The average contribution of each ancestor was thus staled 

 definitely, the contribution diminishing with the remoteness of 

 the ancestor. This Law of Ancestral Inheritance is represented 

 graphically in the accompanying diagram (Fig. 24). Pearson has 

 somewhat modified the figures given by Galton, holding that in 

 horses and dogs the parents contribute 1/2, the grandparents 1/3, 

 the great grandparents 2/9, etc. 



Number of Ancestors. — Theoretically the number of ancestors 

 doubles in each ascending generation; there are two parents, 

 four grandparents, eight great-grandparents, etc. If this con- 

 tinued to be true indefinitely the number of ancestors in any 

 ascending generation would be (2) 11 , in which n represents the 

 number of generations. There have been about 57 generations 

 since the beginning of the Christian Era, and if this rule held true 

 indefinitely each of us would have had at the time of the birth 

 of Christ a number of ancestors represented by (2)" or about 

 120 quadrillions, — a number far greater than the entire human 

 population of the globe since that time. As a matter of fact, 



d 1 



c? 



<3 



8 



cS 



<S 



? 



s 



d 



cS 



6 



? 



6 



cS 



c? 



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o* 



Parents 



Grand Pta 



Gt Gd Pts 



$ Cft G\ G'd P'ta 



Fig. 24. Diagram of Galton's "Law of Ancestral Inheritance." The 

 whole heritage is represented by the entire rectangle; that derived from 

 each progenitor by the smaller squares; the number of the latter doubles in 

 each ascending generation while its area is halved. (After Thompson.) 



