58 NATURAL INHERITANCE. [CHAP. 



is zero. This results from what was said a few pages 

 back about the most probable measure in a Scheme 

 being its M. In a Scheme of Errors the M is equal to 

 0, therefore the most Probable Error in such a Scheme 

 is also. It is astonishing that mathematicians, who 

 are the most precise and perspicacious of men, have not 

 long since revolted against this cumbrous, slip-shod, 

 and misleading phrase. They really mean what I 

 should call the Mid-Error, but their phrase is too firmly 

 established for me to uproot it. I shall however always 

 write the word Probable when used in this sense, in the 

 form of " Prob. " ; thus " Prob. Error," as a continual 

 protest against its illegitimate use, and as some slight 

 safeguard against its misinterpretation. Moreover the 

 term Probable Error is absurd when applied to the 

 subjects now in hand, such as Stature, Eye-colour, 

 Artistic Faculty, or Disease. I shall therefore usually 

 speak of Prob. Deviation. 



Though the value of our Q is the same as that of 

 the Prob. Deviation, Q is not a convertible term with 

 Prob. Deviation. We shall often have to speak of the 

 one without immediate reference to the other, just as 

 we speak of the diameter of the circle without reference 

 to any of its properties, such as, if lines are drawn from 

 its ends to any point in the circumference, they will 

 meet at a right angle. The Q of a Scheme is as de- 

 finite a phrase as the Diameter of a Circle, but we 

 cannot replace Q in that phrase by the words Prob. 

 Deviation, and speak of the Prob. Deviation of a 

 Scheme, without doing some violence to language. We 



