by means of natural selection. In many other cases, modifications
are probably the direct result of the laws of variation or of growth,
independently of any good having been thus gained. But even such
structures have often, as we may feel assured, been subsequently taken
advantage of, and still further modified, for the good of species under
new conditions of life. We may, also, believe that a part formerly of
high importance has frequently 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 means of natural selection.

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 predicated.

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