much doubt and
difficulty in Darwin's mind, although at present it forms one of
the main supports of the prevailing explanation of the origin of new
species. Darwin simply states that this kind of variability seems to
be of a peculiar nature; since polymorphic species are now in a stable
condition their occurrence gives no clue as to the mode of origin of
new species. Polymorphic species are the expression of the result
of previous variability acting on a large scale; but they now simply
consist of more or less numerous elementary species, which, as far as we
know, do not at present exhibit a larger degree of variability than any
other more uniform species. The vernal whitlow-grass (Draba verna) and
the wild pansy are the best known examples; both have spread over almost
the whole of Europe and are split up into hundreds of elementary
forms. These sub-species show no signs of any extraordinary degree
of variability, when cultivated under conditions necessary for the
exclusion of inter-crossing. Hooker has shown, in the case of some ferns
distributed over still wider areas, that the extinction of some of the
intermediate forms in such groups would suffice to justify the elevation
of the remaining types to the rank of distinct species. Polymorphic
species may now be regarded as the link which unites ordinary
variability with the 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
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