ANNUAL MIGRATIONS 117 



Supposing then, for the sake of formulating a 

 working hypothesis, that we adopt the view that 

 day-length provides the environmental stimulus 

 that arouses the autumnal migratory impulse. 

 This in itself is not enough for we know that a bird 

 cannot consciously evaluate day-length. We must 

 therefore leave intellect and deliberation out of the 

 argument. As a substitute for mental we may 

 legitimately postulate physiological control. We 

 have already seen that the reproductive hormones 

 can evoke particular modes of instinctive behaviour 

 and since, according to our definition (p. 46) 

 migration involves reproduction, we are justified in 

 considering it to be a particular phase of sexual 

 behaviour. A gonadal hormone, elaborated at a 

 specific season — the time of migration — might thus 

 provide the stipulated physiological stimulus. 



The suggestion becomes more attractive when we 

 recall that the reproductive organs of birds exhibit 

 a very marked rhythm, synchronous with the migra- 

 tions themselves. The northward journey coin- 

 cides with enlargement: the southward with dimi- 

 nution. The noticeable anatomical changes might 

 well be accompanied by physiological. Appear- 

 ances, at least, support our hypothesis. 



To complete the chain, there would still remain to 

 determine the actual mechanism by which an altera- 



