XV THE PHILOSOPHY OF ZOOLOGY 657 



of Birds, whea the sexes differ, as they do in a large proportion 



of the species, the male has always more brilliant coloration, and 



often possesses also special crests or frills, wattles and the like, 



not present or less developed in the female. The greater obscurity 



of the colouring of the female Bird appears to be adapted to 



rendering her less conspicuous to enemies, such as Birds of Prey, 



while sitting on the nest ; and, in cases where the females are 



brightly coloured, the nest is covered over above, or is constructed 



in a hole in the ground. The brilliant colouring and other 



features distinguishing the males 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, ivarning 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 oi protective resemhlance 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 



