v COLOURS OF ANIMALS 367 
and reptiles, while the chief departure from the rule occurs 
in birds, though even here in very many cases the law of 
sexual likeness prevails. But in all cases where the increas- 
ing development of colour became disadvantageous to the 
female, it would be checked by natural selection, and thus 
produce those numerous instances of protective colouring in 
the female only, which occur most frequently in these two 
groups, birds and butterflies. 
Colour as a means of Recognition 
There is also, I believe, a very important purpose and use 
of the varied colours of the higher animals in the facility it 
affords for recognition by the sexes or by the young of the 
same species; and it is this use which probably fixes and 
determines the coloration in many cases. When differences 
in the size and form of allied species are very slight, colour 
affords the only means of recognition at a distance, or while 
in motion; and such a distinctive character must therefore 
be of especial value to flying insects which are continually in 
motion, and encounter each other, as it were, by accident. 
This view offers us an explanation of the curious fact that 
among butterflies the females of closely-allied species in the 
same locality sometimes differ considerably, while the males 
are much alike; for, as the males are the swiftest and by far 
the highest fliers, and seek out the females, it would evidently 
be advantageous for them to be able to recognise their true 
partners at some distance off. This peculiarity occurs with 
many species of Papilio, Diadema, Adolias, and Colias; and 
these are all genera, the males of which are strong on the 
wing and mount high in the air. In birds such marked 
differences of colour are not required owing to their higher 
organisation and more perfect senses, which render recogni- 
tion easy by means of a combination of very slight differential 
characters. + 
This principle may perhaps, however, account for some 
anomalies of coloration among the higher animals. Thus, 
while admitting that the hare and the rabbit are coloured 
protectively, Mr. Darwin remarks that the latter, while 
1 For numerous examples of recognition-colours in birds, see Darwinism, 
pp. 217-226, 
