ISOLATION AND SEXUAL SELECTION. 85 



fighting weapons, such as the large canine teeth of male 

 mammals and the antlers of stags, is accounted for by 

 the survival of variations leading to victory in the fierce 

 struggles which take place between the males at the 

 breeding season. The correctness of this view can hardly 

 be doubted ; but there is great divergence of opinion 

 when the principle of sexual selection is applied to the 

 more purely ornamental secondary sexual characters. 

 These characters are very commonly developed among 

 the higher animals, and one may mention as examples 

 the mane of the lion and bison, the ornamental patches 

 of colour and hair in monkeys, the beard in man ; the 

 brilliant wattles and plumage in numberless birds, such 

 as the gorgeous feathers of ^pheasants, peacocks, birds of 

 paradise, and humming birds ; the ornamental colours 

 in many fish, butterflies and spiders, the horn-like pro- 

 cesses of beetles, the attractive scents of butterflies, the 

 vocal sounds emitted by insects, frogs, and mammals, 

 and the beautiful song of birds. Darwin pointed out 

 that these brilliant and striking characters appeal to 

 the senses of the female, and are deliberately displayed 

 to her at the breeding season. Courtship with these 

 animals is often a lengthy and elaborate business, during 

 which the male may perform a regular dance and strike 

 attitudes to display himself to the best advantage. The 

 more attractive male will thus secure his mate more 

 rapidly and certainly than the less happily endowed, or, 

 in the case of polygamous species, will be followed by a 

 greater number of females. Even if the less successful 

 males eventually manage to pair, they will not have 

 such good or so many chances of leaving offspring. 

 A small percentage of advantage in this respect is 

 sufficient to bring about a selective result. Now it 

 is characteristic of such structures and colours that 

 they develop only in one sex and generally only at ma- 

 turity ; frequently, as in birds, they are periodically 

 renewed at each breeding season. Moreover, they 



