xv THE PHILOSOPHY OF ZOOLOGY 637 



of many Birds may be in great part the by-product of higher 

 vitality, and may thus be the indirect outcome of natural selection 

 leading to the more vigorous "males obtaining an advantage in 

 contest with rivals. It is possible, also, that the choice of the 

 female in selecting a mate may have been a factor in bringing about 

 the special modifications in question. But the evidence which has 

 been adduced for any such selection on the part of the female of a 

 mate with some slight superiority in brilliancy of colouring, or in 

 the development of crests and the like, over his rivals, is insufficient, 

 and many observations tend to show that selection of this kind, 

 though it may occur, is exceptional. In any case, as a general 

 explanation of secondary sexual characters, sexual selection is not 

 at the present time very widely accepted as adequate. 



Protective and Aggressive Resemblance and Mimicry. 

 One of the most important of the phenomena which are well ex- 

 plained by the theory of natural selection, and which may, therefore, 

 well be taken as affording evidence in favour of that theory, are the 

 phenomena of protective resemblance, warning characters, and of 

 mimicry. In innumerable cases among all classes of animals there 

 are found instances of a resemblance between the animal and its 

 ordinary natural surroundings, which has the effect of rendering it 

 inconspicuous and unlikely to attract the observation of an enemy, 

 or of its prey. Such a resemblance is brought about sometimes 

 merely by colour, very often by the arrangement of the colour in a 

 pattern, this being frequently accompanied by modifications of 

 shape, including sometimes the development of special excrescences 

 or appendages. In some cases of protective resemblance the colour 

 and even the markings change with a change of the surroundings. 

 For details of such cases reference must be made to special works. 

 Many Insects present elaborate markings which give them a close 

 resemblance to a tuft of lichen or moss, a twig, a leaf, or other 

 object, and resemblances of an equally striking character occur in 

 other classes. 



Some animals, more especially certain Insects, are protected by 

 their nauseous character against being devoured by animals that 

 would otherwise prey upon them ; but often, no doubt, such 

 nauseous Insects are attacked and killed before their unpalatable 

 character is detected. It is thus manifestly of advantage to such 

 animals that they should be readily recognisable, and should thus 

 be passed over ; and in many such cases the coloration is bright 

 and conspicuous, or the animal is rendered conspicuous by other 

 means (warning characters). 



By mimicry is meant a superficial resemblance borne by one 

 animal to a member of a different group. The best-known 

 examples of mimicry occur among the Insects. It is manifestly 

 of advantage to a Butterfly belonging to a group which is not 

 nauseous to be readily mistaken for a nauseous form with con- 



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