es advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the fang of
the adder, and in the ovipositor of the ichneumon, by which its eggs are
{201} 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
this snake is furnished with a rattle for its own injury, namely, to warn
its prey to escape. 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. But I have not space here to enter on this and other such cases.

Natural selection will never produce in a being anything injurious to
itself, 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 country with
which it has to struggle for existence. And we see that this is the degree
of perfection attained under nature. The endemic productions of New
Zealand, for instance, are perfect one compared with another; but they are
now rapidly yielding before the advancing legions of plants {202} and
animals introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said,
on high authority, not to be perfect even in that most perfect organ, the
eye. If our reason