84 THE RIDDLE OF MIGRATION 



we may be suggesting a possible mode of origin of 

 instinctive behaviour, but we are still left with 

 the much more difficult problem of how repeated 

 habits can impress themselves on the nervous system 

 and constitution and ultimately the chromosomes. 

 Moreover it is difficult to see how such an explana- 

 tion could apply to many curious instincts in the 

 insect world. 



Subsidiary to this main problem are several minor 

 ones. Even if we are satisfied that topographical 

 memory is the mainspring of homing ability, it can- 

 not apply here. Again we are tempted to substitute 

 magnetic sensibility — the only possible proposition 

 in our present state of knowledge — but on top of the 

 difficulties already considered, we must now add an 

 inheritance factor. How a bird could distinguish a 

 particular part of the magnetic field at all, is un- 

 known. How it could recognise it, or find its way 

 to it, on its first sortie into the wide world, is even 

 more inexplicable. Yet it does go south rather than 

 north or east or west, even if its flights are nocturnal 

 anditcannot possibly be "following the sun." Mag- 

 netic sensibility is an intriguing hypothesis but the 

 best we can say for it is that it has never been dis- 

 proved. There is no evidence in its favor. 



Instinctive behaviour can be modified by experi- 

 ence (p. 87). As far as adult birds are concerned 



