t ichneumonidae feeding within the live bodies of
caterpillars; and at other such cases. The wonder indeed is, on the theory
of natural selection, that more cases of the want of absolute perfection
have not been observed.

The complex and little known laws governing variation are the same, as far
as we can see, with the laws which have governed the production of
so-called specific forms. In both cases physical conditions seem to have
produced but little direct effect; yet when varieties enter any zone, they
occasionally assume some of the characters of the species proper to that
zone. In both varieties and species, use and disuse seem to have produced
some effect; for it is difficult to resist this {473} 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 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