VARIATION AND HEREDITY 



203 



base-line (a-6) ; in the diagram these run from 9 

 to 23. These figures happen to represent the vari- 

 ation in the number of veins in beech leaves, but they 

 might represent the limits of weights of seeds in 

 grams, or the number of spines in a fish's fin, or any 

 other measurable character. The number of variates 

 in each class is represented by the cross-lines, each 



FIG. 74. Two symmetrical curves illustrating the value of " a " as a 

 measure of variability (see text). 



line standing for forty individuals. It will be noted 

 that the greatest number of veins falls on 16, which is 

 very nearly midway between 9 and 23. This middle 

 point is called the mode or mean. Again, there is a 

 point on each half -curve where the curvature changes 

 from concave to convex (point of inflection). Let 

 us now compare the curves in fig. 74. In both of 

 these curves the mode is the same. The higher 

 curve, compared with the lower, shows that the 

 variates in the former array are concentrated, as it 



