case; for almost all our animals and plants have been greatly improved in
many ways within a recent period; and this implies variation. It would be
equally rash to assert that characters now increased to their utmost limit,
could not, after remaining fixed for many centuries, again vary under new
conditions of life. No doubt, as Mr. Wallace has remarked with much truth,
a limit will be at last reached. For instance, there must be a limit to
the fleetness of any terrestrial animal, as this will be determined by the
friction to be overcome, the weight of the body to be carried, and the
power of contraction in the muscular fibres. But what concerns us is that
the domestic varieties of the same species differ from each other in almost
every character, which man has attended to and selected, more than do the
distinct species of the same genera. Isidore Geoffroy St. Hilaire has
proved this in regard to size, and so it is with colour, and probably with
the length of hair. With respect to fleetness, which depends on many
bodily characters, Eclipse was far fleeter, and a dray-horse is comparably
stronger, than any two natural species belonging to the same genus. So
with plants, the seeds of the different varieties of the bean or maize
probably differ more in size than do the seeds of the distinct species in
any one genus in the same two families. The same remark holds good in
regard to the fruit of the several varieties of the plum, and still more
strongly with the melon, as well as in many other analogous cases.

To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and indirectly by
affecting the reproductive system. It is not probable that variability is
an inherent and necessary contingent, under all circumstances. The greater
or less force of inheritance and reversion determine whether variations
shall endure. Variability is governed by many unknown laws, of which
correlated growth is probably the most important. Something, but how much