He who believes that each equine species was independently created, will, I
presume, assert that each species has been created with a tendency to vary,
both under nature and under domestication, in this particular manner, so as
often to become striped like the other species of the genus; and that each
has been created with a strong tendency, when crossed with species
inhabiting distant quarters of the world, to produce hybrids resembling in
their stripes, not their own parents, but other species of the genus. To
admit this view is, as it seems to me, to reject a real for an unreal, or
at least for an unknown cause. It makes the works of God a mere mockery
and deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created in
stone so as to mock the shells now living on the sea-shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case out of
a hundred can we pretend to assign any reason why this or that part has
varied. But whenever we have the means of instituting a comparison, the
same laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between species
of the same genus. Changed conditions generally induce mere fluctuating
variability, but sometimes they cause direct and definite effects; and
these may become strongly marked in the course of time, though we have not
sufficient evidence on this head.
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