74 EVOLUTION OF BIRD- SONG 



mates who appealed to their sense of sight rather 

 than to that of hearing ? But, it may be asked, do 

 birds which live in trees ever depend upon their 

 eyes rather than upon their hearing for the detection 

 of an enemy ? In our temperate climate there are 

 practically no enemies whose approach through trees 

 will not be more readily perceived by ear than by 

 eye ; but in tropical climates there are other foes to 

 reckon with a numerous race of tree serpents, 

 whose approach is silent, yet terribly dangerous, and 

 whose colours often resemble the general tone of 

 their surroundings. These animals are great 

 enemies of birds, who must watch carefully for them, 

 as well as listen for sounds announcing the arrival 

 of other predacious creatures. If such watchfulness 

 would have had the effect of developing and 

 rendering critical the powers of sight, it may have 

 induced birds to prefer mates whose hues appealed 

 particularly to this sense ; and it would thus explain 

 why arboreal birds which live in tropical climates 

 are often so brightly coloured. But the arboreal 

 birds in temperate climates have few or none of 

 these serpent enemies to fear ; hence they depend 

 upon the ear rather than upon the eye for the 

 detection of an enemy, and when hearing no alarm 

 they may be at ease. 



