THE CALL- NOTE 47 



The frequent similarity of the danger-cries and the 

 call-notes among birds physically allied, but whose 

 songs are quite distinct, will be discussed in the 

 chapter treating of the influence of heredity. It 

 may be here mentioned as affording strong evidence 

 of the original development of the call-note from the 

 danger-cry. 



Several influences would have determined the 

 particular characteristics of the cries uttered by 

 different races of animals ; among these, the capacity 

 of the throat, due to a habitual mode of feeding, 

 also an ability to repeat a cry many times with 

 one breath doubtless had some effect ; and in song- 

 birds the tones might have been gradually affected 

 by any recurrent prevalent sounds falling on the 

 receptive senses of successive generations of the 

 young. The perpetuation of characteristic notes of 

 birds may be easily accounted for. The recognition 

 of them by parent birds was of vital importance 

 to the young, whose cries for food would have been 

 effectual in proportion to their vehemence and the 

 certainty of their being understood. But if males 

 uttered songs (by which I mean vocal sounds distinct 

 from alarms and call-notes), they might on this 

 account be preferred, and in this event the call-note 

 would be less frequently employed in song by the 



