repeatedly pointed out in my work on the Mutation Theory. ("Die
Mutationstheorie", 2 vols., Leipzig, 1901.)

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 accumulative
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 equilibrium 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 generations. 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.

The external causes of this curious period of mutability are as yet
wholly unknown and can hardly be guessed at, since the origin of the
Oenothera Lamarckiana is veiled in mystery. The seeds, introduced into
England about 1860, were said to have come from Texas, but whether from
wild or from cultivated plants we do not know. Nor has the species been
recorded as having been observed in the wild condition. This, however,
is nothing peculiar. The European types of Oenothera biennis and O.
muricata are in the same condition. T