226 
DARWINISM 
CHAP. 
oil the head or neck, often not interfering with the generally 
protective character of their plumage. Such are the bright 
patches of blue, red, or yellow, by which the usually green 
Eastern barbets are distinguished; and similar bright patches 
of colour characterise the separate species of small green 
fruit-doves. 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 ixseful, 
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 every¬ 
where, 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. That this is so is shown by the fact that “ the 
common white butterfly often Hies down to a bit of paper on 
the ground, no doubt mistaking it for one of its own species 
while, according to Mr. Collingwood, in the Malay Archipelago, 
“a dead butterfly pinned upon a conspicuous twig will often 
arrest an insect of the same species in its headlong flight, and 
