EXPERIMENTAL EVOLUTION 



not by the prolonged selection of fluctuating variations, 

 but at one step in one generation as "sports" of the 

 wild Brassica oleracea. Strawberry plants without run- 

 ners, green dahlias and green roses, the common seedless 

 bananas of the markets, the Shirley poppies, pitcher- 

 leaved ash trees, Pierson's variety of the Boston fern, 



FIG. 58. Clover leaves with three to nine leaflets, illustrating a tend- 

 ency to mutate. The normal clover leaf is a pinnately compound leaf 

 with three leaflets. Plants with leaves having five to nine leaflets con- 

 stitute a "half-race," i.e. the normal character is active, the anomaly 

 semi-latent. (Photo by the author; specimens from cultures of G. H. 

 Shutl.) 



5-9- "leaved" clovers (Fig. 58), 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. 48), 

 and the innumerable white flowered varieties of colored 

 flowered species, are all illustrations of mutation. Fre- 

 quently the imitative change occurs in a lateral bud, pro- 



