74 THE SENSES OF ANIMALS 



flight. I am not suggesting that the blackbird has "philan- 

 thropic feelings" towards its fellow birds; the point I am 

 making is that the alarm call is understood, so to speak, by 

 the other species. This does not apply to the territorial song 

 of birds of differing kinds. A cock robin will warn off another 

 cock robin who seeks to invade his territory ; but within that 

 territory there may be tits or thrushes to whom the robin's 

 song has no significance at all. 



The same sort of thing can be noted in country where 

 Redshanks are to be found. The redshank has truly been 

 called "the warden of the marshes", and its striking alarm 

 note will cause other meadow and marshland birds to freeze. 

 The redshank would seem to react very quickly indeed to 

 an approaching danger. 



This appears to indicate some kind of discriminating 

 ability in hearing even if it be discrimination of an elemen- 

 tary and limited kind. Among mammals, however, discrimi- 

 nation between one sound and another is very marked ; and, 

 as we shall note later on, a mammal's ability to sort out 

 scents is even more marvellous. 



Much further work on this kind of problem by field 

 naturalists would be well worth while, because although it is 

 obvious that a particular animal may only be able to detect 

 sounds within certain ranges of frequencies, and therefore 

 sounds beyond its own limits are not heard at all, there are 

 many animals which share ranges of sounds, even though 

 they themselves are unrelated. 



Mammals are, on the whole, as much dependent upon 

 their hearing as on sight, and the way in which many of 

 them can pick out certain sounds from a jungle chorus, or 

 remember sounds and attach particular importance to them 

 is a most remarkable feature of their auditory abilities. 



It will be shown in Part II that the external ears of 

 mammals give some indication of their sensitiveness of hear- 

 ing, the Elephant being an extreme example of this ; but 

 there are somewhat puzzling instances where it can be shown 

 that a mammal is very sensitive indeed to sounds although 

 its external ear is neither unduly large nor very mobile. This 

 can be observed in the Water Vole which can react to the 

 slightest sound ; but, as compared with other rodents such as 



