530 STRUCTURE AND LIFE HISTORIES 
the oscillation about this mean illustrates continuous or 
fluctuating variation. 
But we may conceive that the point of suspension of the 
pendulum changes, as shown in the figure. The pendulum 
continues to oscillate, but now about a new mean position; 
a new character has been introduced, with its own fluctua- 
tions of more or less. 
453. Mutations. Darwin, as well as others before and 
after him, recognized both kinds of variation, but de Vries 
was the first to work out in detail the hypothesis that 
discontinuous variations furnish the material for natural 
selection. Discontinuous variations he called mutations; 
plants which give rise to or "throw" them are said to 
mutate. A plant that arises by mutation is an elementary 
species, or mutant; and the theory that mutations (and not 
fluctuations) explain the origin of the fittest, and supply 
the materials upon which natural selection operates in the 
formation of new species, de Vries called the mutation theory. 
454. Examples of Mutation. The kohlrabi, cauli- 
flower, and other horticultural varieties of the wild cliff- 
cabbage (Fig. 397), are believed to be mutants, and to have 
arisen, not by the prolonged selection of fluctuating varia- 
tions, but at one step in one generation as "sports" 
of the wild Brassica oleracea. Strawberry plants without 
runners, green dahlias and green roses, the common seed- 
less bananas of the markets, the Shirley poppies, pitcher- 
leaved ash trees, Pierson's variety of the Boston fern, 
5-9- "leaved" clovers (Fig. 398), white black-birds (and 
other albinos, including albino men), moss-roses, thornless 
cacti and thornless honey-locusts, red sunflowers, com- 
posites with tubular corollas in the ray-flowers (Fig. 399), 
and the innumerable white flowered varieties of colored 
