104 HEREDITY AND EVOLUTION IN PLANTS 



of the species or variety. In other words, continuous or 

 fluctuating variation is variation about a mean. It may 

 be illustrated by the bob of a swinging pendulum, which 

 continually fluctuates within definite limits about the 

 mean position assumed when the pendulum is at rest 



(Fig. 56). 



All plants and animals manifest fluctuating variation 

 in all their characters (Fig. 50), and such variations are 

 largely, if not entirely, dependent upon the environment. 

 A slight change in the kind of food elements supplied, or 

 in the amount of water or sunlight available will make the 

 leaves or petals a deeper or a paler color. Rich soil, fa- 

 voring a more abundant food supply, will cause a greater 

 average growth than poor soil, but unless the seed for 

 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 

 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, or otherwise. When the selection 

 ceases, and the soil is maintained at average fertility, the 

 characteristic average height of the plants is restored. 



85. 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 market, and the lengths of the indi- 



'De Vries. "The Mutation Theory," vol. 2, p.- 47, Chicago, 1909 



