How the sense of beauty in its simplest form--that is, the reception of a peculiar kind of pleasure from certain colours, forms and sounds--was first developed in the mind of man and of the lower animals, is a very obscure subject.

The same sort of difficulty is presented if we enquire how it is that certain flavours and odours give pleasure, and others displeasure.

Habit in all these cases appears to have come to a certain extent into play; but there must be some fundamental cause in the constitution of the nervous system in each species.

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.