te states show that wonderful metamorphoses
in function are at least possible. For instance, a swim-bladder has
apparently been converted into an air-breathing lung. The same organ having
performed {205} simultaneously very different functions, and then having
been specialised for one function; and two very distinct organs having
performed at the same time the same function, the one having been perfected
whilst aided by the other, must often have largely facilitated transitions.

We are far too ignorant, in almost every case, to be enabled to assert that
any part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumulated by
means of natural selection. But we may confidently believe that many
modifications, wholly due to the laws of growth, and at first in no way
advantageous to a species, have been subsequently taken advantage of by the
still further modified descendants of this species. We may, also, believe
that a part formerly of high importance has often been retained (as the
tail of an aquatic animal by its terrestrial descendants), though it has
become of such small importance that it could not, in its present state,
have been acquired by natural selection,--a power which acts solely by the
preservation of profitable variations in the struggle for life.

Natural selection will produce nothing in one species for the exclusive
good or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious to
another species, but in all cases at the same time useful to the owner.
Natural selection in each well-stocked country, must act chiefly through
the competition of the inhabitants one with another, and consequently will
produce perfection, or strength in the battle for life, only according to
the standard of that country. Hence the inhabitants of one country,
generally the smaller one, will often yield, as we see they do yield, to
the inhabitants of another and generally larger country. For in {206} the
larger country there will have existed more individuals, and more
diversified forms, and the competition will have been severer, and thus the
standard of perfection will have been rendered higher. Natural selection
will not necessarily produce absolute perfection; nor, as far as we can
judge by our limited faculties, can absolute perfection be everywhere
found.

On the theory of n