v.] NORMAL VARIABILITY. 55 



accuracy of measurement, and without the slightest idea 

 until the time of Quetelet that they might be applicable 

 to human measures. But Errors, Differences, Deviations, 

 Divergencies, Dispersions, and individual Variations, all 

 spring from the same kind of causes. Objects that bear 

 the same name, or can be described by the same phrase, 

 are thereby acknowledged to have common points of 

 resemblance, and to rank as members of the same species, 

 class, or whatever else we may please to call the group. 

 On the other hand, every object has Differences peculiar 

 to itself, by which it is distinguished from others. 



This general statement is applicable to thousands of 

 instances. The Law of Error finds a footing wherever 

 the individual peculiarities are wholly due to the com- 

 bined influence of a multitude of " accidents," in the 

 sense in which that word has already been defined. 

 All persons conversant with statistics are aware that 

 this supposition brings Variability within the grasp 

 of the laws of Chance, with the result that the 

 relative frequency of Deviations of different amounts 

 admits of being calculated, when those amounts are 

 measured in terms of any self-contained unit of varia- 

 bility, such as our Q. The Tables 4 to 8 give the 

 results of these purely mathematical calculations, and 

 the Curves based upon them may with propriety be 

 distinguished as " Normal." Tables 7 and 8 are based 

 upon the familiar Table of the Probability Integral, 

 given in Table 5, via that in Table 6, in which the unit 

 of variability is taken to be the " Probable Error " or 

 our Q, and not the " Modulus." Then I turn Table 6 



