To this necessity for specialisation in colour, by which each bird may easily recognise its kind, is probably due that marvellous variety in the peculiar beauties of some groups of birds.

The Duke of Argyll, speaking of the humming birds, made the objection that "A crest of topaz is no better in the struggle for existence than a crest of sapphire.

A frill ending in spangles of the emerald is no better in the battle of life than a frill ending in spangles of the ruby.

A tail is not affected for the purposes of flight, whether its marginal or its central feathers are decorated with white;" and he goes on to urge that mere beauty and variety for their own sake are the only causes of these differences.
But, on the principles here suggested, the divergence itself is useful, and must have been produced _pari passu_ with the structural differences on which the differentiation of species depends; and thus we have explained the curious fact that prominent differences of colour often distinguish species otherwise very closely allied to each other.
Among insects, the principle of distinctive coloration for recognition has probably been at work in the production of the wonderful diversity of colour and marking we find everywhere, more especially among the butterflies and moths; and here its chief function may have been to secure the pairing together of individuals of the same species.

In some of the moths this has been secured by a peculiar odour, which attracts the males to the females from a distance; but there is no evidence that this is universal or even general, and among butterflies, especially, the characteristic colour and marking, aided by size and form, afford the most probable means of recognition.