70 EVOLUTION OF BIRD-SONG 



songsters belong to the order of Insessores (or 

 perchers), and their vocal organs are much more 

 complex than those of other birds ; yet it is a 

 singular fact that some of the Insessores, such as 

 ravens, crows, and magpies, possess the proper 

 apparatus (Bishop, in Todd's Cyclop, of Anat, and 

 Phys., vol. iv. p. 1496), though they never sing, and 

 do not naturally modulate their voices to any great 

 extent " (Descent of Man, p. 370). It is probable 

 that the Insessores, and vast numbers of pre-existing 

 species, have always been of arboreal habits, and 

 have thus been dependent on the voice for a means 

 of intercommunication when at some little distance 

 apart, and especially for a means of announcing the 

 approach of an enemy through the thick foliage in 

 which so many of the insectivores spend most of 

 their time. Consequently their voices, and simul- 

 taneously their hearing, would have been gradually 

 developed ; and the latter feature, from having been 

 so continually exercised in the vital necessity of 

 detecting signals of danger, or mere cries of distress, 

 would have become at once delicate, critical, and 

 accurate, both in males and in females ; and the 

 latter would therefore have been competent to detect, 

 and perhaps liable to have been attracted by, any ab- 

 normal powers of melodious utterance in their suitors. 



