THE SIMPLEST SONGS 57 



accompanied by a trembling of the whole of the 

 body, and seems to be the very expression of 

 passion. This movement of the wings in accord- 

 ance with the frequency with which a single cry 

 is repeated may be observed in the yellow bunting, 

 cirl bunting, whitethroat, lesser whitethroat, and 

 several other birds. 



It is not surprising that bird-phrases should be 

 repeated in the same intervals of pitch, or that the 

 distinguishable notes which they include should 

 follow the same orders of succession, as habitually 

 occurs in all the more inferior singers ; for amongst 

 ourselves we detect a corresponding influence of 

 habit in the wearying repetitions of street hawkers 

 and railway porters. Just as loudness of voice may 

 have been, and probably was, a factor in the sur- 

 vival of certain young, so a vehement and forceful 

 exclamation of call-notes uttered by males to females 

 may very possibly have been perpetuated, firstly, 

 by the agency of selection, and, secondly, by the 

 imitativeness of the young. The rivalry and emula- 

 tion of the males, also, may have likewise instigated 

 the introduction of modifications or additions to the 

 phrases ; and these, if agreeable to the females, 

 would probably have been continued in succeeding 

 generations by the agency of imitation. 



