II] VARIATION 13 



as many ordinates and the curve would be nearly 

 smooth. 



It is clear that a curve of this kind can be used 

 for comparing the variability of different characters, 

 for the greater the variability of the population the 

 wider Avill be the base ; consequently the curve for 

 a very variable character will be relatively low and 

 wide, that for a slightly variable one measured in the 

 same scale will be tall and steep. A curve of this 

 kind, which is quite similar on either side of the 

 longest perpendicular ('median,' representing the 

 modal value), may be obtained by plotting any 

 measurements which vary fortuitously around a most 

 frequent value, and such a curve is called a ' normal 

 curve.' For example, if a large number of beans in- 

 cluding equal numbers of white ones and black ones 

 were placed in a sack, and drawn out ten at a time 

 without selection of colour, most frequently five white 

 and five black would be drawn, less often six of one 

 colour and four of the other, more rarely seven and 

 three and so on to the rarest case of ten of one colour. 

 If the numbers of white beans in a draw are plotted 

 along the base-line, and the ordinates represent the 

 number of draws for each combination, a polygon 

 approaching the normal curve will be obtained. 

 Variation which gives a normal curve when plotted 

 in this way is spoken of as normal variation. 



As mentioned above, the steepness of the curve is 



