ancestors. Upon this progressive mutability the main development of the
animal and vegetable kingdom evidently depends. In contrast to this, the
two other cases are called retrogressive and degressive mutability. In
nature retrogressive mutability plays a large part; in agriculture 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 disappearance or
reappearance of some character. With progressive mutations the case is not
so simple, since the new character must first be produced 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. ("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