152 THE FIRST PRINCIPLES OF HEREDITY 



compare the different curves of variability with each other. 

 Now, given the median in a S3mimetrical curve, we can find 

 the measure of variabiHty in any given curve. In com- 

 paring different curves we find that they differ in their 

 steepness — that is, some are flatter with the base more 

 spread out, others steeper (see Fig. 69). Assuming both 

 curves to stand for the same number of observations 

 (which implies that both curves have the same area), the 

 flatter curve with the greater base indicates greater 

 variability, the steeper curve less variability, of the 

 characters plotted. We have therefore in the steepness of 

 the curve a common measure of the variability of both 



Fig. 70. — Curve of Probability, with Median (M), Quartile (Q), 

 AND Standard Derivation (5). (After Vernon.) 



curves by determining the degree of spread of each curve. 

 This can be done in various ways, which it would be too 

 difhcult here to propound in detail. We shall fully 

 explain only one method, that of determining what is called 

 the " Quartile " of the Curve. 



The median, we have said, divides the area of the curve 

 into two equal halves ; now the quartile (Q) is a vertical 

 line on each side of the median, which divides each half- 

 area of the curve into two equal quarters. The distance of 

 this quartile from the median (MQ or MQ^) will be greater 

 in the flatter curve (see Fig. 69), and thus this length MQ 



