historical production of species. But it does not appear that they had
this significance for Darwin; and, in fact, they exhibit no phenomena which
could explain the processes by which one species has been derived from
another. By thus narrowing the limits of the species-producing variability
Darwin was led to regard small deviations as the source from which natural
selection derives material upon which to act. But even these are not all
of the same type, and Darwin was well aware of the fact.

It should here be pointed out that in order to be selected, a change must
first have been produced. This proposition, which now seems self-evident,
has, however, been a source of much difference of opinion among Darwin's
followers. The opinion that natural selection produces changes in useful
directions has prevailed for a long time. In other words, it was assumed
that natural selection, by the simple means of singling out, could induce
small and useful changes to increase and to reach any desired degree of
deviation from the original type. In my opinion this view was never
actually held by Darwin. It is in 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