ii] HEREDITY 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 will 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. If the curve is 

 quite similar on either side of the longest perpen- 

 dicular (' median,' representing the modal value), it is 

 called a ' normal curve,' and such a curve may be 

 obtained by plotting any measurements which vary 

 fortuitously around a most frequent value. For 

 example, if a large number of beans including 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 often 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 



