EXPERIMENTAL EVOLUTION 527 
9 inches the frequency was zero. When the ears were 
arranged in piles according to their length, the tops Of 
the piles indicated the curve of frequency (Fig. 393). 
The curve of frequency indicates the quantitative dis- 
tribution of any character or quality when its occurrence 
is dependent largely upon chance. This is strikingly 
illustrated by the grouping of bean seeds rolled down a 
smooth inclined plane, and collected in receptacles at the 
bottom (Fig. 394). The seeds are started rolling midway 
between the edges of the plane; the chances are about 
equal for some of the seeds to fall into the outside compart- 
ments, but the odds are vastly in favor of their landing at 
or near the center. Thus they group themselves so that 
the tops of the piles form a curve of chance variation. 
When the result is influenced in one direction more than 
in another the crest of the curve will be nearer one extreme 
than the other, and the curve is to that extent skew. The 
curve of bean seeds in Fig. 394 is slightly skew toward 
the right-hand extreme. Suggest one or more reasons why. 
460. Fluctuating Variation and Inheritance. When 
the ancestry is not mixed or hybrid the curve of frequency 
of any character in one generation ordinarily tends to 
recur in successive generations of descendants, 1 providing 
the environment remains essentially the same. 
451. Discontinuous Variation. Long before Darwin, 
students of plants and animals had observed a different 
kind of variation than continuous one which was not 
quantitative but qualitative, resulting in the expression of 
new characters, or of a new curve of frequency; that is, in 
fluctuation about a new mean. Plants from some of the 
1 The behavior of hybrid descendants is a special case described in 
Chapter XXXVII. 
