I90 THE BIOLOGY OF BIRDS 



of the influence of the display upon her ; she is won 

 by it. 



We have given prominence to Mr. Selous' observations 

 because they seem to us to aftord cumulative evidence of 

 the reality of sexual selection in the narrower Dar\vinian 

 sense — of preferential mating on the female's part and on 

 the determination of preference by the male's success in 

 arousing and focussing sex-interest. 



Auks. — The courting guillemots twine their necks 

 together, nibble at one another's plumage, bow profusely 

 to one another, wag their heads from side to side, interlock 

 their bills and sway their bodies. One is often seen running 

 its bill down the front of the breast until it reaches the rock, 

 then jerking it up again. This is like a male's performance ; 

 but, as the sexes are practically identical, it is difficult to 

 tell what part is the cock's initiative and what the hen's 

 response. They seem to be monogamous, and both parents 

 share in brooding (as their worn patches show) and in bring- 

 ing food to the young bird. The same is true of the nearly 

 related razorbills, but they are more vocal in their courtship, 

 and while they resemble guillemots in their billing and 

 nibbling caresses, they do not indulge in the profuse bowing 

 ceremonial. We mention a detail like this because it illus- 

 trates the individuality or specificity of nearly related forms. 

 In the case of puffins, which belong to the same auk family, 

 there is a good deal of fighting among the males at the breed- 

 ing season, and there is billing and bowing between the 

 sexes. The head is often jerked upwards with the yellow 

 mouth wide open. 



§ 6. The Evolution of the Voice 



For many millions of years there was no sound of life 

 upon the earth, and among Invertebrates there is no voice, 

 nothing more than the instrumental music of chirping insects. 

 This is produced by rapidly moving one hard part of the 

 body against another. The first animals to have a voice were 

 Amphibians ; that is to say, they have vocal cords which 



