explained on the ground of a supposed analogy with sports, which are for
the most part of a retrogressive nature.

Excluding all these more or less sudden changes, there remains a long
series of gradations of variability, but all of these are not assumed by
Darwin to be equally fit for the production of new species. In the first
place, he disregards all mere temporary variations, such as size, albinism,
etc.; further, he points out that very many species have almost certainly
been produced by steps, not greater, and probably not very much smaller,
than those separating closely related varieties. For varieties are only
small species. Next comes the question of polymorphic species: their
occurrence seems to have been a source of 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