76 Variation 
and in horticulture it gives rise to numerous varieties, which have in 
the past been preserved, either on account of their usefulness or 
beauty, or simply as fancy-types. In fact the possession of numbers of 
varieties may be considered as the main character of domesticated 
animals and cultivated plants. 
In the case of retrogressive and degressive mutability the internal 
cause is at once apparent, for it is this which causes the disappear- 
ance or reappearance of some character. With progressive mutations 
the case is not so simple, since the new character must first be pro- 
duced and then displayed. These two processes are theoretically 
different, but they may occur together or after long intervals. 
The production of the new character I call premutation, and the 
displaying mutation. Both of course must have their external as 
well as their internal causes, as I have repeatedly pointed out in my 
work on the Mutation Theory’. 
It is probable that nutrition plays as important a part among the 
external causes of mutability as it does among those of fluctuating 
variability. Observations in support of this view, however, are too 
scanty to allow of a definite judgment. Darwin assumed an accumu- 
lative influence of external causes in the case of the production of new 
varieties or species. The accumulation might be limited to the 
life-time of a single individual, or embrace that of two or more 
generations. In the end a degree of instability in the equilibrium of 
one or more characters might be attained, great enough for a character 
to give way under a small shock produced by changed conditions of 
life. The character would then be thrown over from the old state 
of equilibrium into a new one. 
Characters which happen to be in this state of unstable equi- 
librium are called mutable. They may be either latent or active, 
being in the former case derived from old active ones or produced as 
new ones (by the process, designated premutation). They may be 
inherited in this mutable condition during a long series of genera- 
tions. I have shown that in the case of the evening primrose of 
Lamarck this state of mutability must have existed for at least 
half a century, for this species was introduced from Texas into 
England about the year 1860, and since then all the strains derived + 
from its first distribution over the several countries of Europe show 
the same phenomena in producing new forms. The production of 
the dwarf evening primrose, or Oenothera nanella, is assumed to be 
due to one of the factors, which determines the tall stature of the 
parent form, becoming latent; this would, therefore, afford an example 
of retrogressive mutation. Most of the other types of my new 
mutants, on the other hand, seem to be due to progressive mutability. 
1 Die Mutationstheorie, 2 vols., Leipzig, 1901. 
