io6 BIOMETRY 



ever it was enunciated, although Mendel's work was 

 not generally known until later. According to Mendel's 

 theory of inheritance, certain ancestors contribute 

 nothing to the constitution of certain offspring in respect 

 of certain characters. Furthermore, the modification of 

 the law of ancestral heredity which applied to alterna- 

 tive inheritance, and which was assumed in working 

 out the inheritance of coat colour in thoroughbred 

 horses, has recently been shown not to apply to that 

 particular case. 



Unfortunately, most of the further biometrical 

 generalizations are based upon the assumption that 

 the law of ancestral heredity is strictly true. So that 

 whilst we have spent some time in considering the 

 facts of normal variability and of correlation between 

 relatives, because these facts are quite independent of 

 any theoretical assumption, the remainder of our 

 review must be passed over at a more rapid rate. Until 

 the theoretical conclusions now to be described have 

 been revised by their authors in the light of recent 

 knowledge, it is difficult to say how much reliance is to 

 be laid upon them, but it seems quite likely that they 

 will hold good as approximations. Indeed, though 

 not applying to individual cases, the law of ancestral 

 heredity does seem to hold good as a statistical state- 

 ment of general results, so that there would be no objec- 

 tion to it on either theoretical or practical grounds if 

 only it had been enunciated in some such terms as ' a 

 law of average ancestral resemblance.' Thus it is quite 

 possible that the total contribution of the eight great- 

 grandparents of an individual may be on the average 



