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 have
been inherited without change for an immense period.
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