ix ANIMAL BEHAVIOUR 167 



organs or pedicellariae which close on the tube-feet. The 

 starfish then draws away an arm, wrenching off the 

 pedicellarise. In the heart-urchin the pediccllarisc are 

 separated off reflexly. The starfish repeats the process 

 with the same or another arm until the sea-urchin is 

 cleared of its weapons. The starfish then protrudes a 

 portion of its very elastic stomach over its victim, and 

 the business is over. Some items in the process are 

 reflex, but the point is that the starfish exhibits a chain 

 of actions, certainly not in the line of least resistance, 



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which are mutually adjusted or correlated in such a way 

 that they bring about an end an end which is reached 

 not immediately but eventually. In the absence of 

 ganglia we may not speak of intelligent behaviour, but 

 effective behaviour it certainly is. Many of the lo\ver 

 animals, then, show a trial and error method, a selection 

 of the answers-back that put things right, and for a short 

 time, at least, a capacity of profiting by experience. 



8. Instinctive Behaviour. Among the more complex 

 animals there is abundant illustration of reflex and 

 tropistic movements, but the greater part of behaviour 

 is either instinctive or intelligent, or a blend of the two. 

 Instinctive behaviour, such as is seen when a spider 

 makes a web or the bees build a honevcomb, reaches its 



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climax in insects. Intelligent behaviour, such as is 

 illustrated when rooks break the shells of river-mussels 

 by letting them fall from a height, or when collie-dogs 

 learn to anticipate difficulties in bringing the sheep home, 

 reaches a climax in higher Vertebrates, notably birds 

 and mammals. Each of the modes has its particular 

 excellences and limitations, and though they are, to 

 our thinking, on different lines of evolution, they are 

 often found in co-operation. 



As a thoroughly typical instance of instinctive be- 

 haviour we take that of the Yucca Moth (Pronuba yucca- 

 sella) which has been often cited. When the large yellow 

 bells of the yucca open, each for a single night, the 

 silvery moth, just emerged from her chrysalis, sets forth 

 to visit them. From the anthers of one she collects 

 pollen, which she kneads into a ball and holds beneath 



