332 THE BIOLOGY OF BIRDS 



and many birds have a sensitive skin on their bills. This 

 is most marked in birds that probe in the mud or soil for 

 food, such as snipe and woodcock ; they are able by touch 

 to detect the presence of food which they cannot see. The 

 tactility is sometimes especially localised in a swollen 

 white patch near the nostrils, which is called the cere 

 because of its waxy appearance. 



Sense o! Taste. — Most birds eat in a hurry ; it is pre- 

 carious to take time to test and chew. The sense of taste 

 is poorly developed. Perhaps most of them are specialists 

 in diet, keeping to one kind of food which they recognise 

 by sight. In some cases the tongue is very horny, as in 

 parrots, and is used to rasp the pulp of fruits from the stone ; 

 in other cases, as in woodpeckers and humming-birds, the 

 tongue helps to convey the food to the mouth ; in other 

 cases, such as ducks, the tongue is a touch-organ. Only 

 in a few cases does taste appear to be of much importance. 

 Experiments show, however, that hungry birds will reject 

 tempting morsels which have been dipped in some unpalat- 

 able fluid, and there are sometimes taste-papillas on the 

 tongue, that is to say, nerve-endings receptive to substances 

 dissolved in the mouth. 



§ 4. The Inclined Plane of Animal Behaviour 



Every one is agreed that the various forms of animal 

 behaviour are on different levels of complexity, and we 

 have tried to express this in a diagram (Fig. 49). 



The theory behind the diagram is twofold : (i) that 

 as we ascend the series of animal agencies it becomes 

 increasingly necessary to recognise a mental or psychical 

 aspect ; and (2) that animal activities are either predomi- 

 nantly reactive or predominantly initiative, either (in the 

 main) the expression of what is hereditarily enregistered, 

 or (in the main) the expression of new tentatives or 

 experiments. The contrast will become clearer as we 

 proceed. 



The aim of science is to discover descriptive formulse 



