aracteristic of varieties come to be augmented into the greater
differences characteristic of species, and, by the extermination of the
older intermediate forms, new species come to be distinctly defined
objects. Thus, also, we shall see how it is that organic beings can be
classed by what is called a natural method in distinct groups--species
under genera, and genera under families.

As all the inhabitants of each country may be said, owing to their high
rate of reproduction, to be striving to increase in numbers; as each form
is related to many other forms in the struggle for life,--for destroy any
one and its place will be seized by others; as every part of the
organization occasionally varies in some slight degree, and as natural
selection acts exclusively by the preservation of variations which are
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 organization 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 {8} 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
organization. We are almost compelled to look at the specialization 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, organization
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, t