the common law, which states that small
deviations from the mean type are frequent, but that larger aberrations
are rare, the rarer as they are larger. Any degree of variation will
be found to occur, if only the number of individuals studied is large
enough: it is even possible to calculate before hand, how many
specimens must be compared in order to find a previously fixed degree of
deviation.

The variations, which from time to time happen to appear, are evidently
not governed by this law. They cannot, as yet, be produced at will: no
sowings of thousands or even of millions of plants will induce them,
although by such means the chance of their occurring will obviously
be increased. But they are known to occur, and to occur suddenly and
abruptly. They have been observed especially in horticulture, where they
are ranged in the large and ill-defined group called sports. Korschinsky
has collected all the evidence which horticultural literature affords
on this point. (S. Korschinsky, "Heterogenesis und Evolution", "Flora",
Vol. LXXXIX. pages 240-363, 1901.) Several cases of the first appearance
of a horticultural novelty have been recorded: this has always happened
in the same way; it appeared suddenly and unexpectedly without any
definite 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