ws governing the production of varieties
are the same, as far as we can judge, with the laws which have governed
the production of distinct species. In both cases physical conditions
seem to have produced some direct and definite effect, but how much
we cannot say. Thus, when varieties enter any new station, they
occasionally assume some of the characters proper to the species of that
station. With both varieties and species, use and disuse seem to have
produced a considerable effect; for it is impossible to resist this
conclusion when we look, for instance, at the logger-headed duck, which
has wings incapable of flight, in nearly the same condition as in the
domestic duck; or when we look at the burrowing tucu-tucu, which is
occasionally blind, and then at certain moles, which are habitually
blind and have their eyes covered with skin; or when we look at the
blind animals inhabiting the dark caves of America and Europe. With
varieties and species, correlated variation seems to have played an
important part, so that when one part has been modified other parts have
been necessarily modified. With both varieties and species, reversions
to long-lost characters occasionally occur. How inexplicable on the
theory of creation is the occasional appearance of stripes on the
shoulders and legs of the several species of the horse-genus and of
their hybrids! How simply is this fact explained if we believe that
these species are all descended from a striped progenitor, in the same
manner as the several domestic breeds of the pigeon are descended from
the blue and barred rock-pigeon!

On the ordinary view of each species having been independently created,
why should specific characters, or those by which the species of the
same genus differ from each other, be more variable than the generic
characters in which they all agree? Why, for instance, should the colour
of a flower be more likely to vary in any one species of a genus, if
the other species possess differently coloured flowers, than if all
possessed the same coloured flowers? If species are only well-marked
varieties, of which the characters have become in a high degree
permanent, we can understand this fact; for they have already varied
since they branched off from a common progenitor in certain characters,
by which they have come to be specifically distinct from each other;
therefore these same characters would be more likely again to vary than
the generic characters which h