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.
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