commonly in 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 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. Habit in
producing constitutional peculiarities, and use in strengthening, and
disuse in weakening and diminishing organs, appear in many cases to have
been potent in their effects. Homologous parts tend to vary in the same
manner, 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 nouri