seen until adult life, but
which are none the less innate. The offspring of the same parents may
show decided differences, although they are put under similar
conditions, and such differences are of course inherent in the nature of
the individual. Such variations are called _congenital variations_.

There is, however, a second class of variations which are not born in
the individual, but which arise as the result of some conditions
affecting its after-life. The most extreme instances of this kind are
mutilations. Some men have only one leg because the other has been lost
by accident. Here is a variation acquired as the result of
circumstances. A blacksmith differs from other members of his race in
having exceptionally large arm muscles; but here, again, the large
muscles have been produced by use. A European who has lived under a
tropical sun has a darkened skin, but this skin has evidently been
darkened by the action of the sun, and is quite a different thing from
the dark skin of the dark races of men. In such instances we have
variations produced in individuals as the result of outside influences
acting upon them. They are not inborn, but are secondarily acquired by
each individual. We call them _acquired variations_.

It is not always possible to distinguish between these two types of
variation. Frequently a character will be found in regard to which it is
impossible to determine whether it is congenital or acquired. If a child
is born under the tropical sun, how can we tell whether its dark skin
was the result of direct action of the sun on its own skin, or was an
inheritance from its dark-skinned parents? We might suppose that this
could be answered by taking a similar child, bringing it up away from
the tropical sun, and seeing whether his skin remained dark. This would
not suffice, however; for if such a child did then develop a white skin,
we could not tell but that this lighter-coloured skin had been produced
by the direct bleaching effect of the northern climate upon a skin
which otherwise would have been dark. In other words, a conclusive
answer can not here be given. It is not our purpose, however, to attempt
to distinguish between these two kinds of variations, but simply to
recognize that they occur.

Our next problem must be to search for an explanation of these
variations. With the acquired variations we have no particular trouble,
for they are easily explained as due to the direct action of the
env