52 2 STRUCTURE AND LIFE HISTORIES 



447. Two Klinds of Variation. — One of the first results 

 of de Vries's painstaking work was the demonstration of 

 what he believed to be a fundamental difference between 

 two distinct kinds of variation — continuous (or fluctuating) 

 and discontinuous (or saltative, i.e., leaping). 



448. Continuous Variation. — Continuous variation is 

 quantitative — a case merely of more or less. It deals with 

 averages. Some flowers on a red-flowered plant may be 

 lighter or darker red, but, in a series of generations, the 

 average of a large number in each generation does not 

 vary, and the departure from the average never exceeds 

 certain limits. The flowers of a given species may have a 

 certain characteristic odor, but the odor may be stronger 

 in some flowers than in others, or in some individual 

 plants than in others. The plants grown from a handful of 

 beans of the same variety may vary in height within limits , 

 but the average height of a large number will not vary in 

 successive generations, and will be characteristic of the 

 species or variety. In other words, continuous or fluc- 

 tuating 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. 396). 



All plants and animals manifest fluctuating variation 

 in all their characters (Fig. 391), 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, 

 favoring a more abundant food supply, will cause a greater 

 average growth than poor soil, but unless the seed for 



