EXPERIMENTAL STUDY OF HEREDITY 559 
either appearing in their fullness, or not appearing at all. 
From more careful observations we know that such is 
not the case. A blossom may, for example, be more or 
less pink, an odor more or less strong, dwarfs are not 
all the same height, but fluctuate around a mean. We 
conclude therefore that characters do not behave as 
units, and that the conception of "unit-characters" is 
erroneous. The evidence does, however, seem to justify 
the conclusion that the factor or factors, whatever they 
may be, 1 that are causally related to the given character 
do behave as units. We may therefore designate them 
as character-units. They are commonly known as genes. 
Quite probably, in many if not all cases, more than one 
gene or character-unit is involved in the production of 
any given character. 
481. Applications of Mendel's Law. Over 100 pairs 
of structural and color characters have been found, in 
plant breeding, to behave more or less closely in accord- 
ance with the Mendelian conception. In peas alone over 
20 pairs of characters are expressed in successive genera- 
tions, in accordance with this law. Among the more 
striking results which are explainable upon Mendelian 
theory are the following: 
1 . Mottled beans have been produced in the FI genera- 
tion by crossing two varieties, neither of which had mottled 
seeds. Various types appeared in the 2 generation. 
2. Jet black beans have appeared in the FI generation 
from a cross between two varieties, one of which had pure 
white seeds, the other light yellow. Various shades and 
colors appeared in the Fz generation. 
3. In one case three distinct varieties of beans, breeding 
1 Substance or condition, we know not what, within the germ-cells. 
