Chap. VI. WHAT NATURAL SELECTION CAN DO. 201 
sisted in tlie 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 selec¬ 
tion. 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 any¬ 
thing 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 modi¬ 
fied ; 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 en¬ 
demic productions of New Zealand, for instance, are 
perfect one compared with another; but they are now 
rapidly yielding before the advancing legions of plants 
K 3 
