relation to previously existing variability. Dwarf types are one of the
commonest and most favourite varieties of flowering plants; they are not
originated by a repeated selection of the smallest specimens, but appear at
once, without intermediates and without any previous indication. In many
instances they are only about half the height of the original type, thus
constituting obvious novelties. So it is in other cases described by
Korschinsky: these sports or mutations are now recognised to be the main
source of varieties of horticultural plants.

As already stated, I do not pretend that the production of horticultural
novelties is the prototype of the origin of new species in nature. I
assume that they are, as a rule, derived from the parent species by the
loss of some organ or quality, whereas the main lines of the evolution of
the animal and vegetable kingdom are of course determined by progressive
changes. Darwin himself has often pointed out this difference. But the
saltatory origin of horticultural novelties is as yet the simplest parallel
for natural mutations, since it relates to forms and phenomena, best known
to the general student of evolution.

The point which I wish to insist upon is this. The difference between
small and ever present fluctuations and rare and more sudden variations was
clear to Darwin, although the facts known at his time were too meagre to
enable a sharp line to be drawn between these two great classes of
variability. Since Darwin's time evidence, which proves the correctness of
his view, has accumulated with increasing rapidity. Fluctuations
constitute one type; they are never absent and follow the law of chance,
but they do not afford the material from which to build new species.
Mutations, on the other hand, only happen to occur from time to time. They
do not necessarily produce greater changes than fluctuations, but such as
may become, or rather are from their very nature, constant. It is this