MUTATIONS 265 
factor mutation has a certain effect upon every character in the organism, 
yet the visible effects of some factor differences are restricted to a single 
character. According to their visible effects, therefore, we recognize two 
classes of factor mutations: (1) those conditioning apparently only single 
characters; (2) those having a visible manifold effect on the soma. Cages 
Fie. 110.—A seedling of the oak-like walnut (left) and of the California black walnut, 
the parent species (right.) 
involving mutations of the second class are known in several species of 
animals and plants. An interesting example is the oak-like walnut, 
Juglans californica var. quercina, which appears to differ from the parent 
species by a single factor difference, Fig. 110. But this variety is distinct 
from the species type in nearly all gross morphological characters, 
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