the young than in the old. Call the breeds of pigeons, some of which
have bred true for centuries, species; and how exactly parallel is the
case with that of the species of the horse-genus! For myself, I venture
confidently to look back thousands on thousands of generations, and
I see an animal striped like a zebra, but perhaps otherwise very
differently constructed, the common parent of our domestic horse,
whether or not it be descended from one or more wild stocks, of the ass,
the hemionus, quagga, and zebra.

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 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
differs, more or less, from the same part in the parents. 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. The external conditions of life, as climate and food, etc.,
seem to have induced some slight modifications. Habit in producing
constitutional differences, and use in strengthening, and disuse in
weakening and diminishing organs, seem to have been more potent in their
effects. Homologous parts tend to vary in the same way, and homologous
parts tend to cohere. Modifications in hard parts and in external parts
sometimes affect softer and internal parts. When one part is largely
developed, perhaps it tends to draw nourishment from the adjoining
parts; and every part of the structure which can be saved without
detriment to the