of species. Certain variations constantly occur, especially such as are
connected with size, weight, colour, etc. They are usually too small for
natural selection to act upon, having hardly any influence in the struggle
for life: others are more rare, occurring only from time to time, perhaps
once or twice in a century, perhaps even only once in a thousand years.
Moreover, these are of another type, not simply affecting size, number or
weight, but bringing about something new, which may be useful or not.
Whenever the variation is useful natural selection will take hold of it and
preserve it; in other cases the variation may either persist or disappear.

In his criticism of miscellaneous objections brought forward against the
theory of natural selection after the publication of the first edition of
"The Origin of Species", Darwin stated his view on this point very
clearly:--"The doctrine of natural selection or the survival of the
fittest, which implies that when variations or individual differences of a
beneficial nature happen to arise, these will be preserved." ("Origin of
Species" (6th edition), page 169, 1882.) In this sentence the words
"HAPPEN TO ARISE" appear to me of prominent significance. They are
evidently due to the same general conception which prevailed in Darwin's
Pangenesis hypothesis. (Cf. de Vries, "Intracellulare Pangenesis", page
73, Jena, 1889, and "Die Mutationstheorie", I. page 63. Leipzig, 1901.)

A distinction is indicated between ordinary fluctuations which are always
present, and such variations as "happen 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