n to arise" from time to
time. ((I think it right to point out that the interpretation of this
passage from the "Origin" by Professor de Vries is not accepted as
correct either by Mr Francis Darwin or by myself. We do not believe that
Darwin intended to draw any distinction between TWO TYPES of variation;
the words "when variations or individual differences of a beneficial
nature happen to arise" are not in our opinion meant to imply a
distinction between ordinary fluctuations and variations which "happen
to arise," but we believe that "or" is here used in the sense of ALIAS.
With the permission of Professor de Vries, the following extract is
quoted from a letter in which he replied to the objection raised to his
reading of the passage in question:

"As to your remarks on the passage on page 6, I agree that it is now
impossible to see clearly how far Darwin went in his distinction of the
different kinds of variability. Distinctions were only dimly guessed at
by him. But in our endeavour to arrive at a true conception of his view
I think that the chapter on Pangenesis should be our leading guide,
and that we should try to interpret the more difficult passages by that
chapter. A careful and often repeated study of the Pangenesis hypothesis
has convinced me that Darwin, when he wrote that chapter, was well aware
that ordinary variability has nothing to do with evolution, but that
other kinds of variation were necessary. In some chapters he comes
nearer to a clear distinction than in others. To my mind the expression
'happen to arise' is the sharpest indication of his inclining in this
direction. I am quite convinced that numerous expressions in his book
become much clearer when looked at in this way."

The statement in this passage that "Darwin was well aware that ordinary
variability has nothing to do with evolution, but that other kinds
of variation were necessary" is contradicted by many passages in the
"Origin". A.C.S.)) The latter afford the material for natural selection
to act upon on the broad lines of organic development, but the first
do not. Fortuitous variations are the species-producing kind, which the
theory requires; continuous fluctuations constitute, in this respect, a
useless type.

Of late, the study of variability has returned to the recognition of
this distinction. Darwin's variations, which from time to time happen
to arise, are MUTATIONS, the opposite type being commonly designed
fluctuations. A l