EXPERIMENTAL EVOLUTION 



109 



FIG. 54. Curves of variation, illustrating the difference between 

 fluctuation about a given mean, and the appearance of a new mean, i.e., 

 mutation. At the right, variations injthe stature of Oenothera LamarcHana; 

 at the left, variation in the stature of Oenothera nanella, a form derived 

 from O. Lamarckiana by mutation. (After de Vries.) 



FIG. 55. Leaves of varieties of the Boston fern (Nephrolepis), showing 

 (from left to right) progressive branching of the pinna? and pinnules, and 

 illustrating so-called "orthogenetic saltation." (After R. C. Benedict.) 



