524 STRUCTURE AND LITE 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^ 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 11 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^ 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. ThisTcurve 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. 



2 So named from its discoverer, Quetelet (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." 



