150 THE BIOLOGY OF THE SEASONS 



finds its highest expression, and perhaps there is no region 

 so highly favoured as Middle Europe, with its nightingale 

 and bullfinch, its blackbird and thrush, its lark and wren. 

 North America is not far behind with its bobolink and blue- 

 bird. It must be admitted that there are good singers in 

 many parts of the tropics, but they do not attain the domin- 

 ance characteristic of those in the North, and their songs 

 are apt to be drowned in the shrill screams of their neigh- 

 bours. In some warm countries, such as Brazil, the evolu- 

 tion of bird-song seems to have lagged far behind the 

 evolution of gorgeous plumage one of the general facts 

 which suggest the familiar interpretation of song as a 

 factor in preferential mating. 



In mammals, the seat of the voice is in the larynx, at 

 the top of the windpipe ; in birds, the vocal cords are not 

 in the larynx, but in a special song-box or syrinx at the 

 foot of the windpipe, where it divides into the two bronchial 

 tubes. Let us consider the mechanism very briefly. Three 

 parts have to be distinguished. There is the framework 

 of bone, consisting of several pieces, more or less movable 

 in relation to one another. There is the external muscula- 

 ture, moving the parts of the framework and controlled by 

 a rather complex innervation (implicating the twelfth 

 cranial nerve, the first cervical from the spinal cord, and 

 the sympathetic system in the neck). Thirdly, there are 

 vibrating membranes and elastic folds, which are kept in 

 varied degrees of tension by the action of the muscles, 

 so that various notes are produced by the air driven out 

 from the lungs. 



It is of interest to note that the voice-box is most com- 

 plex in Passerine birds, and that within the wide limits 

 of this order there are many grades of differentiation, 

 especially in the masculature. Even among the true 

 songsters (the Passeres seines) there are many grades of 



