CHAPTER VI. DIFFICULTIES OF THE THEORY.
9. SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS OF EXISTENCE EMBRACED BY THE THEORY OF NATURAL SELECTION. (continued)
Natural selection can 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 possessor.
In each well-stocked country natural selection acts through the competition
of the inhabitants and consequently leads to success in the battle for
life, only in accordance with the standard of that particular country.
Hence the inhabitants of one country, generally the smaller one, often
yield to the inhabitants of another and generally the 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 lead to absolute perfection; nor, as far as we can
judge by our limited faculties, can absolute perfection be everywhere
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 to the present inhabitants alone of the world, is
not strictly correct; but if we include all those of past times, whether
known or unknown, it must on this theory be strictly true.
It is generally acknowledged that all organic beings have been formed on
two great laws--Unity of Type, and the Conditions of Existence. By unity
of type is meant that fundamental agreement in structure which we see in
organic beings of the same class, and which is quite independent of their
habits of life. On my theory, unity of type is explained by unity of
descent. The expression of conditions of existence, so often insisted on
by the illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting the varying
parts of each being to its organic and inorganic conditions of life; or by
having adapted them during past periods of time: the adaptations being
aided in many cases by the increased use or disuse of parts, being affected
by the direct action of external conditions of life, and subjected in all
cases to the several laws of growth and variation. Hence, in fact, the law
of the Conditions of Existence is the higher law; as it includes, through
the inheritance of former variations and adaptations, that of Unity of