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 
O 
O 
e@) 
[ \ OO 
[ \ Coe 
OOl 
eee 
‘i \ eoeee 
/ X eee 
A N eooee® 
i OCOCCOCO 
B 
at 
fi i: an 
] 
i =O 
x 
iN fe 
f P cE 
Cc D 
Fic, 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 
