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 



