v.] NORMAL VARIABILITY. 57 



am satisfied to claim that the Normal Curve is a fair 

 average representation of the Observed Curves during 

 nine-tenths of their course ; that is, for so much of 

 them as lies between the grades of 5 and 95. In 

 particular, the agreement of the Curve of Stature with 

 the Normal Curve is very fair, and forms a mainstay of 

 my inquiry into the laws of Natural Inheritance. 



It has already been said that mathematicians laboured 

 at the law of Error for one set of purposes, and we 

 are entering into the fruits of their labours for another. 

 Hence there is no ground for surprise that their Nomen- 

 clature is often cumbrous and out of place, when applied 

 to problems in heredity. This is especially the case 

 with regard to their term of " Probable Error," by which 

 they mean the value that one half of the Errors exceed 

 and the other half fall short of. This is practically the 

 same as our Q. 1 It is strictly the same whenever the 

 two halves of the Scheme of Deviations to which it 

 applies are symmetrically disposed about their common 

 axis. 



The term Probable Error, in its plain English inter- 

 pretation of the most Probable Error, is quite mis- 

 leading, for it is not that. The most Probable Error 

 (as Dr. Venn has pointed out, in his Logic of Chance) 



1 The following little Table may be of service : 



Values of the different Constants when the Prob. Error is taken as unity, and 

 their corresponding Grades. 



Prob. Error I'OOO ; corresponding Grades 25'0, 75 '0 



Modulus 2-097 ; 7'9, 92'l 



Mean Error M83 ; 21 '2, 7 8 '8 



Error of Mean Squares 1'483 ; 16-0, 84'0 



