animals. Thus pigs have occasionally been born with a sort of proboscis,
and if any wild species of the same genus had naturally possessed a
proboscis, it might have been argued that this had appeared as a
monstrosity; but I have as yet failed to find, after diligent search, cases
of monstrosities resembling normal structures in nearly allied forms, and
these alone bear on the question. If monstrous forms of this kind ever do
appear in a state of nature and are capable of reproduction (which is not
always the case), as they occur rarely and singly, their preservation would
depend on unusually favourable circumstances. They would, also, during the
first and succeeding generations cross with the ordinary form, and thus
their abnormal character would almost inevitably be lost. But I shall have
to return in a future chapter to the preservation and perpetuation of
single or occasional variations.

INDIVIDUAL DIFFERENCES.

The many slight differences which appear in the offspring from the same
parents, or which it may be presumed have thus arisen, from being observed
in the individuals of the same species inhabiting the same confined
locality, may be called individual differences. No one supposes that all
the individuals of the same species are cast in the same actual mould.
These individual differences are of the highest importance for us, for they
are often inherited, as must be familiar to every one; and they thus afford
materials for natural selection to act on and accumulate, in the same
manner as man accumulates in any given direction individual differences in
his domesticated productions. These individual differences generally
affect what naturalists consider unimportant parts; but I could show, by a
long catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the most