232 THE PLAY 0F ANIMALS. 



But we ask, What then is the cause of these phe- 

 nomena, if there is no choice by the female? How do 

 the beautiful colours and characteristic forms of male 

 birds arise? Wallace answers these questions as fol- 

 lows: In the first place, it is not at all strange that ani- 

 mals should have colour. In all Nature colour is the 

 rule, black and white are exceptions.* " The presence of 

 some colour, or even of many brilliant colours, in ani- 

 mals and plants would require no other explanation 

 than does that of the sky or the ocean, of the ruby or 

 the emerald — that is, it would require a purely physical 

 explanation only." f The kind of colours, however, is 

 principally determined by natural selection. Colouring 

 for offence and defence is very important in the animal 

 world, a principle which was clearly recognised before 

 the time of this misleading idea of sexual selection. 

 Other peculiarities, such as broad white bands and white 

 or coloured spots, J serve as distinguishing marks to 

 those that live in companies.* 



These marks are important not only in times of 

 danger, when they make it easier for the young to fol- 

 low the old ones, but they also form a kind of bond 

 for the social life, and in addition to that probably serve 

 a useful purpose in hindering the cross-breeding of 

 closely related species. The symmetrical marking which 

 renders the individual recognisable from either side 

 seems to be for this purpose, as we conclude from the 

 facility with which it is lost in domestication. To the 

 same origin may be attributed the characteristic call 



* Nat, Selection and Trop. Nature, 1891, p. 359. 

 f Darwinism, p. 189. 



\ Many such marks are only visible while the animal is in mo- 

 tion, because they would expose it to danger when at rest (ibid.). 



* Like the tribal marks of savages. 



