Origin of Species Variorum

host of magnificently coloured butterflies and some other insects, have been rendered beautiful for beautys sake; but this has been effected not for the delight of man, but through sexual selection, that is from the more beautiful males having been continually preferred by their less ornamented females. So it is with the music of birds. We may infer from all this that a similar taste for beautiful colours and for musical sounds runs through a large part of the animal kingdom. When the female is as beautifully coloured as the male, which is not rarely the case with birds and butterflies, the cause simply lies in the colours acquired through sexual selection having been transmitted to both sexes, instead of to the males alone. In some instances, however, the acquirement of conspicuous colours by the female may have been checked through natural selection, on account of the danger to which she would thus have been exposed during incubation. ↑ ↑

Natural selection cannot possibly produce any modification in any one species exclusively for the good of another species; though throughout nature one species incessantly takes advantage of, and profits by, the structure 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