CHAPTER VIII 
VARIABILITY OF A SINGLE CHARACTER! 
Critical study of a single character . Types - Plotting the frequency curve - 
The mean: The typical individual. Variability or deviation from type- 
Average deviation - Standard deviation - Coefficient of variability - Suggestions 
as to taking measurements - Suggestions as to grouping - Suggestions as to 
numbers - Suggestions as to taking samples - Advantages of statistical studies 
Critical study of a single character. We have seen that the 
individual and the race are made up of an intimate association 
of semi-independent units called characters. Now, owing to the 
differences in heredity and to the vicissitudes of development 
these characters are themselves, in many cases at least, highly 
inconstant, and it remains to study next the variability of a 
single unit character considered by itself alone. 
Suppose we are to study corn characters one by one, as, for 
example, the length of ears. We find at once that different ears 
differ greatly in this respect. How, then, shall we describe this 
character so long as it is not uniform in different ears? We can 
do it only by first ascertaining the ¢yfe, and next learning what 
is the varzabzlity or deviation from this type with respect to 
length, for, of course, variabilities differ in different characters 
even in the same species. It is the business of the present 
chapter to show how this may be done. 
For this purpose take at random, that is, just as they come 
from the field, a lot of ears, say, 300 or thereabouts. Next de- 
cide upon a scale or “scheme” of measurements for grouping. 
1 For a more extended study see “ Principles of Breeding,” chap. xii. 
2 It needs some practice in order to decide upon the most desirable scheme 
for any particular study. It is found that for length half-inch differences give 
as good results as do finer measurements, but that differences of one inch 
fail to give a smooth distribution. With half-inch differences the distribution 
is “ smooth,” that is, the numbers increase and decrease gradually. 
: 105 
