MIGRATION 169 



actions. When a crumb of bread threatens to go down 

 the windpipe instead of the gullet there is an involuntary 

 cough, which is due to pre-arrangements between certain 

 nerve-cells — sensory, intermediate, and motor — and certain 

 muscle-cells. These pre-arrangements are part of the 

 hereditary organisation. So it is with instinctive behaviour, 

 but there is a chain of reflex arcs, and a succession of stimuli 

 may be required. Moreover, many naturalists find good 

 reason for believing that a piece of instinctive behaviour is 

 often suffused with awareness and backed by endeavour. 

 There is unanimity, however, in the conclusion that the 

 neuro-muscular pre-arrangements, which form part of the 

 hereditary structure, require liberating stimuli to set them at 

 work. These stimuli may be from within or from without 

 or from both. Our question concerns the liberating stimuli 

 that lead to migration. 



When eels that have been feeding and growing in a 

 pond for a number of years reach a certain degree of repro- 

 ductive maturity, a change takes place in their blood, thus 

 there is more carbon-dioxide in it than is usual. The 

 creatures become restless and they make their way from 

 pond to river, from river to sea. An internal constitutional 

 change serves as the liberating stimulus which sets the eels 

 on their journey, and there are external stimuli, notably of 

 temperature and pressure, which keep them going until 

 they find the spawning area. Similarly, though we know 

 almost nothing about it, the changes involved in the repro- 

 ductive maturity of birds in spring pull the trigger of the 

 impulse which leads them to leave their winter-quarters 

 and make for their northern breeding places. This will 

 not apply in the same degree to the immature birds, which 

 also migrate. But there are external or environmental 

 stimuli which also operate as trigger-pullers, notably the 

 heat, the glare, and the drought. We must avoid the error 

 of thinking that the liberating stimuli are causes in the 

 sense that the impact of the cue is the cause of the move- 

 ment of the billiard ball, they are causes in the sense that 

 the releasing of the spring of a gramophone is the cause of 



