have been acquired, it will be easy to see if regard be had to
the two following laws, which observation has always confirmed.

FIRST LAW.--In every animal which has not arrived at the term of
its developments, the more frequent and sustained use of any organ
strengthens, develops, and enlarges that organ, and gives it a
power commensurate with the duration of this employment of it. On
the other hand, constant disuse of such organ weakens it by
degrees, causes it to deteriorate, and progressively diminishes its
faculties, so that in the end it disappears.

SECOND LAW.--All qualities naturally acquired by individuals as the
result of circumstances to which their race is exposed for a
considerable time, or as a consequence of a predominant employment
or the disuse of a certain organ, nature preserves to individual
offspring; provided that the acquired modifications are common to
the two sexes, or, at least, to both parents of the individual
offspring.

Naturalists have observed that the members of animals are adapted to
their use, and thence have concluded hitherto that the formation of the
members has led to their appropriate employment. Now, this is an error.
For observation plainly shows that, on the contrary, the development of
the members has been caused by their need and use; that these have
caused them to come into existence where they were wanting.

But let us examine the facts which bear upon the effects of employment
or disuse of organs resulting from the habits which a race has been
compelled to form.

_II.--The Penalties of Disuse_

Permanent disuse of an organ as a consequence of acquired habits
gradually impoverishes it, and in the end causes it to disappear, or
even annihilates it altogether.

Thus vertebrates, which, in spite of innumerable particular
distinctions, are alike in the plan of their organisation, are generally
armed with teeth. Yet those of them which by circumstances have acquired
the habit of swallowing their prey without mastication have been liable
to leave their teeth undeveloped. Consequently, the teeth have either
remained hidden between the bony plates of the jaws, or have even been,
in the course of time, annihilated.

The whale was supposed to have no teeth at all till M. Geoffrey found
them hidden in the jaws of the foetus. He has also found in birds the
groove in which teeth might be placed, but withou