the
long-lost character, and that this tendency, from unknown causes, sometimes
prevails. And we have just seen that in several species of the horse-genus
the stripes are either plainer or appear more commonly in the young than in
the old. Call the breeds of pigeons, some of which have bred true for {167}
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 {168} climate and food, &c., 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