tend to change the various parts of
these living bodies.

So long as the _ranunculus aquatilis_, the water buttercup, is under
water its leaves are all finely indented, and the divisions are
furnished with capillaries; but as soon as the stalk of the plant
reaches the surface the leaves, which develop in the air, are broadened
out, rounded, and simply lobed. If the plant manages to spring up in a
soil that is merely moist, and not covered with water, the stems will be
short, and none of the leaves will show these indentations and
capillaries. You have then the _ranunculus hederaceus_, which botanists
regard as a distinct species.

Among animals changes take place more slowly, and it is therefore more
difficult to determine their cause. The strongest influence, no doubt,
is that of environment. Places far apart are different, and--which is
too commonly ignored--a given place changes its climate and quality with
time, though so slowly in respect of human life that we attribute to it
perfect stability. Hence it arises that we have not only extreme
changes, but also shadowy ones between the extremes.

Everywhere the order of things changes so gradually that man cannot
observe the change directly, and the animal tribes in every place
preserve their habits for a long time; whence arises the apparent
constancy of what we call species--a constancy which has given birth in
us to the idea that these races are as old as nature.

But the surface of the habitable globe varies in nature, situation, and
climate, in every variety of degrees. The naturalist will perceive that
just in proportion as the environment is notably changed will the
species change their characters.

It must always be recognised:

(1) That every considerable and constant change in the environment of a
race of animals works a real change in their wants.

(2) That every change in their wants necessitates new actions to supply
them, and consequently new habits.

(3) That every new want calling for new actions for its satisfaction
affects the animal in one of two ways. Either it has to make more
frequent use of some particular member, and this will develop the part
and cause it to increase in size; or it must employ new members which
will grow in the animal insensibly in response to the inward yearning to
satisfy these wants. And this I will presently prove from known facts.

How the new wants have been able to attain satisfaction, and how the new
habits