Natural 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.
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