xv THE PHILOSOPHY OF ZOOLOGY 619 



or -less widely divergent character in the male and female. 

 Between the males of animals of many groups, contests frequently 

 take place, and this affords us an explanation of the presence or 

 special development in many cases in that sex of various offensive 

 and defensive weapons horns, tusks, and the like. Similarly, we 

 are able to understand the greater vigour, in the majority of cases, 

 of the male, with concomitant greater intensity of coloration, and 

 the development of various ornaments and excrescences not 

 present in the female. In many groups of Insects, and in a large 

 proportion of Birds, sexual differences in coloration are very 

 marked. These are, in some instances, to be traced to the necessity 

 for different protective resemblances required in the two sexes 

 owing to different habits, or to the necessity for protective colora- 

 tions and markings in the female and not in the male. In the 

 case of Birds, when 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 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. 



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 art 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 



