CAUSE AND ORIGIN OF MIGRATION 27 



removed from the winter quarters. The bird re- 

 turns home. 



But here is a serious difficulty urged by some 

 writers as a powerful argument against the sexual 

 impulse as the great factor in the return journey. 

 Many of the birds which migrate northwards or 

 homewards are sexually immature, and others of 

 them are undoubtedly to be classed as ' non- 

 breeders/ 3 which means that during that particular 

 summer they will not be engaged in the work of 

 reproduction ; why, then, should young birds or non- 

 breeders migrate from the winter base. Possibly in 

 the early days of migration only the mature birds 

 did return ; that we cannot state one way or the 

 other. But it is reasonable to argue that once a 

 regular migration habit has become not only con- 

 firmed by heredity but a very true advantage to 

 the species, its influence will be felt by each and 

 every individual. Again it is clear that the sexual 

 impulses, in an undeveloped form, are appreciated 

 by the adolescent, and in many animals by even 

 the most juvenile. The play of all young animals 

 is either an imitation or reflection of the search for 

 food the hunting instinct or the love-making and 

 sexual quarrels pertaining to reproduction, the pre- 

 tended competition by the young for the favours of 

 the opposite sex. They may play at and actually 

 perform a migration which is so closely bound up 



