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 constancy 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, it is in
fullest harmony with the great principle laid down by Darwin. In order
to be acted upon by that complex of environmental forces, which Darwin
has called natural selection, the changes must obviously first be there.
The manner in which they are produced is of secondary importance and has
hardly any bearing on the theory of descent with modification. ("Life
and Letters" II. 125.)

A critical survey of all the facts of variability of plants in nature as
well as under cultivation has led me to the conviction, that Darwin was
right in stating that those rare beneficial variations, which from time
to time happen to arise,--the now so-called mutations--are the real
source of progress in the whole realm of the organic world.

II. EXTERNAL AND INTERNAL CAUSES OF VARIABILITY.

All phenomena of animal and plant life are governed by two sets of
causes; one of these is external, the other internal. As a rule the
internal causes determine the nature of a phenomenon--what an organism
can do and what it cannot do. The external causes, on the other hand,
decide when a certain variation will occur, and to what extent its
features may be developed.

As a very clear and wholly typical instance I cite the cocks-combs
(Celosia). This race is distinguished from allied forms by its faculty
of producing the well-known broad and much twisted combs. Every single
individual possesses this power, but all individuals do