11] 



VARIATION 



15 



modal value, but the average or meau might be about 

 six ; the curve would rise steeply to the mode, and 

 fall away more gradually to the maximum number 

 (fig. 3). Such a curve is described as 'skew.' In 



\ 



500 



•400 



>> 

 u 



a 

 g 300 



IT 

 V 



200 



O 1 2 3 4 5 6 7 8 9 10 11 1? 13 14 15 16 17 18 19 20 21 22 23 2425 



Size of Family 

 Fig. 3. Curve showing distribution of size of 3837 families in 

 America containing deaf-mutes. (After Schuster, ' Hereditary 

 Deafness.' Biometrika, Vol. iv. 1906, p. 474.) 



extreme cases the mode is at one end of the curve, 

 when variation takes place only on one side of it, 

 e.g. in the marsh-marigold, the most frequent number 

 of 'petals' is five, but there may rarely be six, seven 

 or eight, but practically never less than five, so that 

 in plotting the frequency a ' half-curve ' is obtained 

 (fig. 4). 



