THE LAW OF ANCESTEAL HEREDITY 



By KARL PEARSON, F.R.S. 



(1) To any orie wbo has made a close study of heredity, whether it be in raen, 

 mice or plants, there are one or two couclusions which nuist be accepted at ouce. 

 The first of these couclusions is : 



(a) That a knowledge of tlie characters of the parents does not accurately define 

 the character in the offspring. 



Every one is familiär with the fact that given two pairs of parents possessing 

 the like character in the same degree the offspring of one pair will differ from 

 those of the other in respect of this character, and further that all the offspring of 

 the same pair are not alike in the character ; and this Variation in the offspring 

 of the same pair or of like pairs of parents may be very great indeed. 



This point although familiär is frequently neglected. It is so important that 

 it seems worth while to illustrate it by material at my disposal. The pedigrees I 

 give are only samples of many hundreds in my possession, but they are conchisive. 

 I turn first to coat-colour in thoroughbred horses. 



(i) The mare Alhj (bay) covered with Little John (bay) gave a roan colt. 

 There are no black coats in the ancestry of either Ally or Little John back to 

 their great-grandsires and great-grandams. Jest Ally's dam has, however, the 

 Delpini grey coloiir, which aceounts for the roan. Ally again covered, this time 

 by Interpreter (bay), gives a hlack filly. There is no black blood in Ally, but 

 Interpreter s paternal grandsiro was the well-known black horse, Socerer. Thus 

 two bay horses may give a roan, or two bays may give a black foal. I can eite 

 any number of instances, of course, in which they give bay, brown or chestnut 

 offspring. 



* The foUowing paper is put together not as a reply to Mr W. Bateson's rhetorical attack on the Law 

 of Ancestral Heredity (piililished in his recent book Hendel's Priiiriples of Heredity), but simply 

 to indicate to those interested in the matter what are really the fundamental assumptions involved 

 in the Law of Ancestral Heredity, and how far it euables us to describe actually observed experienee in 

 man, horse and dog, which I am unable under any hypothesis to bring under Mendel's "Principles." In 

 the course of this year various investigations on heredity in both plant and animal lifo will I hope be 

 published, and these will tend to throw further light on the laws of heredity. 



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