breathing lung.
The same organ having performed 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 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 natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look only to the