EXPERIMENTAL EVOLUTION 
523 
future generations is selected from the tallest plants, 
and the richness of the soil is maintained, the plants will 
revert to their normal, lower average of height. In other 
words, the average height of the plants of any given variety 
is a constant (unvarying) character, except that it may be 
FIG. 391. Branch of Brachychiton diver sifolium, illustrating fluctuating 
or continuous variation in the shape of the leaves on one plant. 
temporarily altered by careful selection of seeds from the 
tallest or shortest individuals, or by choosing the largest 
or the smallest seeds from any given plant, or by making 
the soil richer or poorer. When the selection ceases, and 
the soil is maintained at average fertility, the characteristic 
average height of the plants is restored. 
