v COLOUKS 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. 1 



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. 



