8S THE RIDDLE OF MIGRATION 



independent of experience ; which tend to the well- 

 being of the individual and the preservation of the 

 race; which are due to the cooperation of external 

 and internal stimuli ; which are similarly performed 

 by all the members of the same more or less re- 

 stricted group of animals; but which are subject to 

 variation and to subsequent modification under the 

 guidance of experience." 



If we have satisfied ourselves that this is the cor- 

 rect attitude to adopt towards migration we can 

 proceed to analyse the implications involved. Two 

 readily separable aspects are concerned and for the 

 sake of clearness we can consider them independently. 



When birds go south they do so at a specific time 

 of year which varies from one species to another but 

 is more or less constant for individuals of a given 

 species in a given locality. There must evidently 

 be one or more stimuli — external or internal or both 

 — that set the wheels in motion, so to speak, and 

 start the southward flight at the appropriate time. 

 Whatever the stimuli, they recur year after year 

 and invariably elicit a similar response. They are 

 annual, something of the moment and therefore 

 something tangible that should, theoretically at 

 least, be amenable to scientific analysis. 



On the other hand, the series of coordinated reflex 

 actions called forth by the stimuli, resulting in the 



