incapable of flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucutucu, 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. In both varieties and
species correlation of growth seems to have played a most important
part, so that when one part has been modified other parts are
necessarily modified. In both varieties and species reversions to
long-lost characters occur. How inexplicable on the theory of creation
is the occasional appearance of stripes on the shoulder and legs of the
several species of the horse-genus and in their hybrids! How simply is
this fact explained if we believe that these species have descended
from a striped progenitor, in the same manner as the several domestic
breeds of pigeon have descended from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently created,
why should the 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, supposed to have been created
independently, have differently coloured flowers, than if all the
species of the genus have 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;
and therefore these same characters would be more likely still to be
variable than the generic characters which have been inherited without
change for an enormous period. It is inexplicable on the theory of
creation why a part developed in a very unusual manner in any one
species of a genus, and therefore, as we may naturally infer, of great
importance to the species, should be eminently liable to variation;
but, on my view, this part has undergone, since the several species
branched off from a common progenitor, an unusual amount of variability
and modification, and therefore we might expect this part generally to
be still variable. But a part may be developed in the most unusual
manner, like the wing of a bat, and yet not be more variable than any
other structure, if the part be common to many subordinate forms, that
353