THE RIDDLE OF MIGRATION 141 



general principles that have suggested themselves 

 can be applied to migrations in other parts of the 

 world. 



It has been seen that the avian brain is of com- 

 paratively lowly organisation, showing many rep- 

 tilian affinities and that intelligence of mammalian 

 level cannot be expected in birds. It was therefore 

 found necessary to assume that natural selection 

 has worked on a passive organism that has re- 

 sponded more or less without being aware of its 

 response. Migrations would as certainly have been 

 evolved by such a method — given sufficient time 

 and material — as by an intelligent understanding 

 of the environment on the part of the bird. 



The custom of migration having been established 

 at some time during the history of any given migra- 

 tory species, it is now assumed to be inherent and 

 to be evoked at the appropriate seasons annually 

 by certain stimuli, external and internal. For 

 reasons given in considerable detail, variations in 

 day-length are assumed to be the primary external 

 stimulus. Experiments in which juncos have been 

 principally used have largely substantiated this 

 viewpoint. 



The internal stimulus has been assumed to be a 

 hormone produced by the interstitial tissue of the 

 reproductive organs. It has been experimentally 



