n contradiction with the acknowledged aim of all his work,--the
explanation of the origin of species by means of natural forces and
phenomena only. Natural selection acts as a sieve; it does not single
out the best variations, but it simply destroys the larger number of
those which are, from some cause or another, unfit for their present
environment. In this way it keeps the strains up to the required
standard, and, in special circumstances, may even improve them.

Returning to the variations which afford the material for the
sieving-action of natural selection, we may distinguish two main kinds.
It is true that the distinction between these was not clear at the time
of Darwin, and that he was unable to draw a sharp line between them.
Nevertheless, in many cases, he was able to separate them, and he often
discussed the question which of the two would be the real source of
the differentiation 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 "happe