Chap. VI. WHAT NATURAL SELECTION CAN DO. 201 



sited 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 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 



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