INHERITANCE IN MANKIND 57 



completely, and appear to be but the chance variations 

 of a homogeneous stock. 



The normal curve must be applied with great 

 caution, but its usual accordance with observation 

 shows that its general features are common to many 

 kinds of statistics. With a fairly homogeneous section 

 of the community, we shall probably not err in looking 

 for many men about the average, and few diverging far 

 from it, whether we examine physical, mental, or moral 

 qualities. 



Galton has applied the theory of variation to the 

 statistical study of ability in man. By a careful analysis, 

 he estimates the number of men of what he defines to 

 be eminent ability as about 250 in each million of our 

 population. He then divides the people into eight 

 classes above mediocrity and eight below it, and takes 

 the three highest classes for the eminent men. Assuming 

 that the normal curve of error applies, the number of 

 men in each class can be calculated. More than half- 

 of each million are placed in the two mediocre classes, 

 one on each side of the line of average ability. As we 

 pass away from the average, the number in a class 

 rapidly diminishes, till the last three classes on each 

 side contain together only about 250 in a million. On 

 one side this number represents the men of eminence ; 

 on the other it is about enough to include the more 

 hopeless idiots and imbeciles. An eminent man departs 

 from mediocrity on one side as far as an idiot does on 

 the other. 



The application of the normal curve to the study 

 of special ability seems justified by the results obtained 

 with examinations. Nevertheless, the candidates in 



