VARIABILITY CURVE FOLLOWING LAW OF CHANCE 
Photograph of beans rolling down an inclined plane and accumulating in c« ympart- 
ments at the base which are closed in front by glass. The exposure was 
long enough to cause the moving beans to appear as caterpillar-like objects 
hopping along the board. If we assume that the irregularity of shape of 
the beans is such that each may make jumps either toward the right or 
toward the left in rolling down the board, the laws of chance lead us to 
expect that in very few cases will these jumps be all in the same direction 
as indicated by the few beans collected in the compartments at the extreme 
right and left. Rather the beans will tend to jump in both right and left 
directions, the most probable condition being that in which the beans make 
an equal number of jumps to the right and to the left as shown by the large 
number accumulated in the central compartment. If the board be tilted 
to one side, the curve of beans would be altered by this one-sided influence. 
In like fashion a series of factors—either of environment or of heredity—if 
acting equally in both favorable and unfavorable directions, will cause a 
collection of ears of corn to assume a similar variability curve when classified 
according to their relative size. Such curves are used by biometricians in 
classifying and studying variations in plants and animals. Photograph 
by A. F. Blakeslee, Carnegie Station for Experimental Evolution, Cold 
Spring Harbor, L. I. (Fig. 18.) 
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