14 HEREDITY [ch. 



a measure of variability, and this can be expressed 

 by taking a point in the curve, the perpendicular 

 from which to the base-line divides the area, enclosed 

 by the curve, the median and the base-line, into two 

 equal parts. Or, differently expressed, the perpen- 

 dicular divides the curve in such a way that the 

 number of individuals between it and the mean is 

 the same as that between it and the extreme. The 

 distance of this perpendicular may be used as a 

 measure of the variability of the character considered, 

 for clearly the greater variability (and thus the flatter 

 the curve), the further this perpendicular will be 

 from the median 1 . 



In most variable characters, the frequency of 

 variation below the mode is not exactly equal to 

 that above it, in which case the curve will be steeper 

 on one side of the mode than on the other, and the 

 average value for the character ( ' mean ') will not be 

 identical with the mode. For example, if the varia- 

 tion in the number of children in a family were 

 plotted in this way, the sizes of families would range 

 from to about 20, but the most frequent number 

 would perhaps be four. Four would then be the 



1 In practice, not this perpendicular, but another rather further 

 from the median is used, which for practical purposes is more con- 

 venient. The distance of this perpendicular, measured in units of 

 the horizontal scale, is called the 'standard deviation" and is regu- 

 larly employed as a measure of variability. 



