176 EVOLUTION OF BIRD-SONG 



two species at least, goldfinch and linnet, every note 

 is acquired by imitation, or rather, that none is 

 inherited. I think we are justified in this general 

 conclusion from the behaviour of only two or three 

 individuals, for the cries of young birds are practi- 

 cally invariable specifically, nor do they vary indi- 

 vidually except in one or two species ; and we find 

 also that the house-sparrow, greenfinch, and bullfinch 

 learn all of their songs by imitation, and inherit 

 none of them. How different in this respect are 

 these birds from the rasorial birds, or from ducks, 

 which, as we have seen, utter the same cries respect- 

 ively, whether they have been reared artificially or 

 naturally ! 



The receptivity of young singing-birds must be 

 very great ; but we should remember what a stimulus 

 that faculty receives from the necessities of the 

 young — their constantly recurring hunger, appeased 

 by the parent, which often at the moment of feeding 

 them utters some call or alarm-note ; and their chilli- 

 ness, removed by the arrival of the brooding mother. 

 These influences are doubly powerful when the young 

 are still blind, for then the feeble creatures can only 

 by the sense of hearing perceive the approach of 

 their helpers. 



The fact that most of the imitative birds above 



