here may arise between individuals of the same
species, of which some are placed in favourable, others amid
unfavourable, conditions, a difference which by degrees becomes very
notable.

Suppose that circumstances keep certain individuals in an ill-nourished
or languid state. Their internal organisation will at length be
modified, and these individuals will engender offspring which will
perpetuate the modifications thus acquired, and thus will in the end
give place to a race quite distinct from that of which the individual
members come together always under circumstances favourable to their
development.

For instance, if a seed of some meadow flower is carried to dry and
stony ground, where it is exposed to the winds and there germinates, the
consequence will be that the plant and its immediate offspring, being
always ill-nourished, will give rise to a race really different from
that which lives in the field; yet this, none the less, will be its
progenitor. The individuals of this race will be dwarfed; and their
organs, some being increased at the expense of the rest, will show
distinctive proportions. What nature does in a long time we do every day
ourselves. Every botanist knows that the vegetables transplanted to our
gardens out of their native soil undergo such changes as render them at
last unrecognisable.

Consider, again, the varieties among our domestic fowls and pigeons, all
of them brought into existence by being raised in diverse circumstances
and different countries, and such as might be sought in vain in a state
of nature. It is matter of common knowledge that if we raise a bird in a
cage, and keep it there for five or six years, it will be unable to fly
if restored to liberty. There has, indeed, been no change as yet in the
form of its members; but if for a long series of generations individuals
of the same race had been kept caged for a considerable time, there is
no room for doubt that the very form of their limbs would little by
little have undergone notable alteration. Much more would this be the
case if their captivity had been accompanied by a marked change of
climate, and if these individuals had by degrees accustomed themselves
to other sorts of food and to other measures for acquiring it. Such
circumstances, taken constantly together, would have formed insensibly a
new and clearly denned race.

The following example shows, in regard to plants, how the change of
some important circumstance may