i68 THE BIOLOGY OF BIRDS 



enreglstered memory. What brings it into operation is an 

 instinctive impulse, associated with internal rhythms of 

 constitution and external periodicities in the environment. 



Evidence that the migratory custom is of the nature 

 indicated may be found in the occurrence of analogous 

 phenomena in many organisms at veiy different levels of 

 mentality — e.g. in land-crabs, in eels (where there is no 

 return journey after reproduction), in salmon, in turtles, 

 in seals. The relative measure of success that attends the 

 adventure of migration, even when it is exhibited by the 

 entirely inexperienced, points to the kind of behaviour 

 we call instinctive. The impulse is inborn and the capacity 

 of effectively obeying it is a ready-made power. Restless- 

 ness at the appropriate time is exhibited by caged birds 

 (not cage-birds, like canaries) which have been made as 

 comfortable as the misguided kindness of man can devise. 

 It is not suggested that birds may not call intelligence to the 

 assistance of their instinct, or that the power of successfully 

 migrating may not be improved in the individual lifetime, 

 but the trend of the evidence is towards the view that the 

 impulse to migrate is instinctive and that the capacity of 

 considerable efficiency in obeying this instinct is also part 

 of the racial inheritance. In any case we must not think 

 of North Temperate migrants flying south at the end of 

 summer because of any definite prevision of the winter. 

 They know no winter in their year, as the poet accurately 

 observes, and have never known any, unless in the case of 

 partial and incipient migrants. It is not by taking thought 

 that these 



" wild birds change their season in the night 

 And wail their way from cloud to cloud 

 Down the long wind." 



§ 4. Immediate Stimuli liberating the Migratory 

 Impulse 



A piece of instinctive behaviour is conditioned on its 

 physiological side by a concatenated series of neuro-muscular 

 pre-arrangements, comparable to those in simple reflex 



