524 STRUCTURE AND LIFE HISTORIES 
449. Illustrations of Continuous Variation. In a quart 
of beans, for example, there are no two seeds of precisely 
the same proportion or size; some are longer, some shorter. 
De Vries describes 1 an experiment in which about 450 
beans were chosen from a quantity purchased in the mar- 
ket, and the lengths of the individuals measured. The 
length varied from 8 to 16 millimeters, and in the following 
proportions : 
Millimeters 8 9 10 n 12 13 14 15 16 
Number of beans .... i 2 23 108 167 106 33 7 i 
The beans were then placed in a glass jar divided into nine 
compartments, all the beans of the same length in the 
same compartment. When this was done it was found 
that the beans were so grouped that the tops of the columns 
in the various compartments followed a curve, known as 
Quetelet's 2 curve (Fig. 392). 
This curve may be plotted by erecting vertical lines 
(ordinates) at intervals of i millimeter on a horizontal line 
or base, the height of each vertical line being proportionate 
to the number of beans having the length indicated in 
figures at its base. This curve shows the frequency of 
occurrence of seeds of any given dimension between the 
two limits or extremes, and is therefore often referred to as 
a curve of frequency. It should be noted that, in the case 
illustrated, the greatest frequency (indicated by the high- 
est point of the curve) very nearly coincides with the aver- 
age dimension; in other words, the more any given character 
1 De Vries. "The Mutation Theory," vol. 2, p. 47, Chicago, 1909. 
1 So named from its discoverer, Qu6telet (Ket-lay). As de Vries 
states: "For a more exact demonstration a correction would be necessary, 
since obviously the larger beans fill up their compartment more than a 
similar number of small ones." 
