528 STRUCTURE AND LIFE HISTORIES 
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 new 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. 
