22 HEREDITY 



called, is fairly common, and mathematical expressions 

 have been invented to measure its amount. 



Sometimes, too, variabiUty curves show two or more 

 distinct summits, wliich generally indicates that the 

 group of organisms in question is separating into two 

 or more distinct types. 



The form of the variability curve, then, gives us a 

 good indication of the kind of variabihty that occurs. 

 But it would evidently be desirable to have some con- 

 venient measure of the amount of variabihty. 



We might, for example, state the fuU range of varia- 

 tion. We might say that, in the case quoted before, 

 the stature of the men measured varied between fifty- 

 3QQ . eight and seventy-seven inches. This, 



however, would be an unsatisfactory ex- 

 pression of the amount of variabihty, 

 since it is very much a matter of chance 

 whether the extreme types occur or not. 

 What is wanted is some figure which 

 would be a sort of average of all the in- 

 dividual differences from the mean. It 

 seems somewhat paradoxical at first 

 sight to speak of the average deviation 

 from the mean or general average, but on 

 consideration it will be evident that such 

 an expression may easily be determined. In doing 

 this, one does not consider the differences from the 

 mean as plus and minus, but proceeds simply to add 

 them all together, and to take the average. 



Tliis may be made clearer by means of an example. 

 In an actual case,i the lengths of 185 nuts were measured, 

 and the measurements arranged in groups as follows. 

 The lengths are given in milhmetres, and each class 

 includes all individuals falhng M-itliin two milhmetres 

 of the figure given. Thus the " 42 " class includes aU 

 that fall between 40 and 44. 



Length SO 34 38 42 46 50 54 58 



Number of nuts . . 2 7 28 59 49 33 6 1 



1 From Elderton, A Primer of Statistics, p. 23. 



