26 EVOLUTION OF BIRD-SONG 



variation, if only in degrees of vehemence, and con- 

 sequently in length or rapidity of repetition, in these 

 cries. The blackbird affords an example of such an 

 occurrence. Its alarm is very like the alarms of its 

 allies the ring-ouzel, mistle-thrush, and fieldfare, but 

 is uttered more frequently, more rapidly, and with 

 greater variety of pitch than any alarms of those 

 species ; and sometimes, especially towards evening 

 in February and March, it appears to be employed 

 for the mere purpose of making a noise and attract- 

 ing attention. The influence of heredity in per- 

 petuating alarm-cries will be discussed in later pages. 

 The relation of particular alarm-cries of birds to 

 the presence of certain enemies is a deeply interest- 

 ing subject, which has not received much attention 

 from ornithologists. Many birds, when approached 

 by enemies, express vocally, as well as in other ways, 

 various degrees of fear, which indicate a considerable 

 amount of discrimination. The expression of fear 

 may have been inherited, as it is in the domestic 

 cock, who yet retains the prominent habit of utter- 

 ing a yell of alarm when any large bird unex- 

 pectedly flies over him, a habit exhibited by his 

 remote ancestor (Gallus bankivd] in India. 1 Mr. 



1 It is worthy of remark that often, when first the young cockerel 

 attempts to crow, he utters a simple yell, closely similar to the alarm- 

 yell of the adult bird. 



