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 oi Brachychiton diver sijolmm, 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. 



