118 THE RIDDLE OF MIGRATION 



tion in day-length could regulate the production 

 of a hormone, a distinctly complex problem. For 

 the moment we can do no better than postpone its 

 consideration. 



Before proceeding, let us recapitulate the argu- 

 ment. Northern migrations were evolved and 

 became established as respective species independ- 

 ently invaded the north. The process may have 

 taken many centuries, but as we see them today 

 migrations are thousands, perhaps millions of years 

 old and fall into the category of instinctive be- 

 haviour. Just as other instincts lie dormant till 

 some external stimulus elicits response, so with the 

 migratory passage. By elimination, on various 

 grounds, we have selected day-length as the most 

 plausible environmental stimulus and have assumed 

 that it controls the developmental condition of the 

 gonads. We have further assumed that a hormone, 

 which provides the physiological stimulus to migra- 

 tion, is elaborated when the gonads are in a par- 

 ticular phase of their cycle, i.e. either increasing 

 or decreasing. 



It was in the hope of ultimately finding some way 

 of applying the experimental method to the subject 

 of migration, that a study of the field aspects, par- 

 ticularly those obtaining in the northern hemisphere, 

 was begun a good many years ago. The conception 



