Variation and Heredity 



263 



single individual standing far removed from the rest, so that 

 a polygon containing such an individual might appear to 

 show greater variation than really exists. Therefore we need 

 a measure of variability that shall take into account the 



C D 



Fig. 3. — Curves of frequency, etc. A, normal curve. B, showing the method 

 of arranging individuals in lines containing similar kinds of individuals. 

 C, curve that is skew to the right. D, polygon of frequencies of horns of 

 rhinoceros beetles. (After Davenport.) 



departures of all the individuals from the mode. One such 

 measure is the arithmetical average of all the departures from 

 the mean in both directions ; and this measure has been 

 widely employed. At present another method is preferred, 

 namely, the square root of the squared departures. This 

 measure is called the standard deviation. The standard 



