522 STRUCTURE ANT> LIFE HISTORIES 
447. Two Kinds 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 
