CHAPTER VIII 



VARIABILITY OF A SINGLE CHARACTER i 



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 tj'pe, and next learning what 

 is the variability 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.^ 



* Yox a more extended study see '* Principles of Hreeding," chap. xii. 



* It needs some practice in order to decide upon the most desiraljie 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 

 U " smooth," that is, the numbers increase and decrease gradually. 



105 



