Fluctuations and Mutations 73 
duced at will: no sowings of thousands or even of millions of planta 
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', 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 recog- 
nised to be the main source of varieties of horticultural plants. 
As already stated, I do not pretend that the production of horti- 
cultural 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 con- 
stancy which is the mark of specific characters, and on this basis every 
new specific character may be assumed to have arisen by mutation. 
Some authors have tried to show that the theory of mutation is 
opposed to Darwin's views. But this is erroneous. On the contrary, 
1 §. Korschinsky, ‘“Heterogenesis und Evolution,” Flora, Vol. uxxxrx. pp. 240—363, 1901. 
