196 TROPICAL NATURE, AND OTHER ESSAYS. 



marked differences in this respect. With the exception 

 of butterflies, the sexes are almost alike in the great 

 majority of insects. The same is the case in mammals 

 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 increasing development of colour became disadvan- 

 tageous 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 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 size and form 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 continu- 

 ally 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 



