tural selection cannot possibly produce any modification in a species
exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by the
structures of others. But natural selection can and does often produce
structures for the direct injury of other animals, as we see in the fang
of the adder, and in the ovipositor of the ichneumon, by which its eggs
are deposited in the living bodies of other insects. If it could be
proved that any part of the structure of any one species had been
formed for the exclusive good of another species, it would annihilate my
theory, for such could not have been produced through natural selection.
Although many statements may be found in works on natural history to
this effect, I cannot find even one which seems to me of any weight. It
is admitted that the rattlesnake has a poison-fang for its own defence
and for the destruction of its prey; but some authors suppose that at
the same time it is furnished with a rattle for its own injury, namely,
to warn its prey. I would almost as soon believe that the cat curls the
end of its tail when preparing to spring, in order to warn the doomed
mouse. It is a much more probable view that the rattlesnake uses its
rattle, the cobra expands its frill and the puff-adder swells while
hissing so loudly and harshly, in order to alarm the many birds and
beasts which are known to attack even the most venomous species. Snakes
act on the same principle which makes the hen ruffle her feathers and
expand her wings when a dog approaches her chickens. But I have not
space here to enlarge on the many ways by which animals endeavour to
frighten away their enemies.

Natural selection will never produce in a being any structure more
injurious than beneficial to that being, for natural selection acts
solely by and for the good of each. No organ will be formed, as Paley
has remarked, for the purpose of causing pain or for doing an injury
to its possessor. If a fair balance be struck between the good and evil
caused by each part, each will be found on the whole advantageous. After
the lapse of time, under changing conditions of life, if any part comes
to be injurious, it will be modified; or if it be not so, the being will
become extinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than the other inhabitants of the same countr