528 STRUCTURE AND LIFE HISTOIUES 



seeds of a red-flowered specimen bear flowers, not that 

 vary from deeper to paler red, but that suddenly, at one step, 

 have become pure white; one or more seeds from an odor- 

 less plant may give rise to individuals whose flowers are 

 sweet-scented ; or vice versa, odorless specimens may spring 

 at one leap, not by gradual minute changes, from those that 



Fig. 395. — Leaves of varieties of the Boston fern (Nepkrolepis), showing 

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

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



are fragrant; in one generation the factors controlling height 

 are so altered that, in successive generations, the average 

 of height may change by either more or less, so that the 

 heights of the individuals fluctuate about a neu< mean. In 

 other words, we recognize a second type of variation — not 

 the fluctuation of individuals about an unchanging mean, 

 but the appearance of a new mean, about which the given 

 character in individuals may fluctuate. 



