advantageous under the excessively complex conditions to which each being
is exposed, no limit exists to the number, singularity, and perfection of
the contrivances and co-adaptations which may thus be produced. An animal
or a plant may thus slowly become related in its structure and habits in
the most intricate manner to many other animals and plants, and to the
physical conditions of its home. Variations in the organisation will in
some cases be aided by habit, or by the use and disuse of parts, and they
will be governed by the direct action of the surrounding physical
conditions and by correlation of growth.

On the principles here briefly sketched out, there is no innate or
necessary tendency in each being to its own advancement in the scale of
organisation. We are almost compelled to look at the specialisation or
differentiation of parts or organs for different functions as the best or
even sole standard of advancement; for by such division of labour each
function of body and mind is better performed. And as natural selection
acts exclusively through the preservation of profitable modifications of
structure, and as the conditions of life in each area generally become more
and more complex from the increasing number of different forms which
inhabit it and from most of these forms acquiring a more and more perfect
structure, we may confidently believe, that, on the whole, organisation
advances. Nevertheless a very simple form fitted for very simple conditions
of life might remain for indefinite ages unaltered or unimproved; for what
would it profit an infusorial animalcule, for instance, or an intestinal
worm, to become highly organised? Members of a high group might even
become, and this apparently has often occurred, fitted for simpler
conditions of life; and in this case natural selection would tend to
simplify or degrade the organisation, for complicated mechanism for simple
actions would be useless or even disadvantageous.