86 THE RIDDLE OF MIGIL\TION 



harvest of eastern Canada was first discovered by 

 accident, through wandering, that the discoverers 

 came to make annual use of their knowledge and in 

 time all adults, after deserting their young, took 

 the east-south road to the Argentine. This route 

 would thus represent a comparatively recent in- 

 novation, a modification of instinctive behaviour 

 based on experience, and on memory perpetuated by 

 precept from generation to generation. Underlying 

 it is the inherent urge to go south to the Argentine. 

 To summarize: Homing is a return to a place 

 already familiar. In a majority of cases sight and 

 topographical memory alone suffice to account for 

 the performance. But there are cases, like that of 

 the penguin, to which such a simple explanation 

 could not apply. That these birds can detect and 

 make use of the variations in the magnetic field is 

 a possible suggestion but one that must, in the 

 present state of knowledge, be considered entirely 

 hypothetical. In the case of many young birds, 

 migrating for the first time and alone, topographical 

 memory can play no part. If we invoke magnetic 

 sensibility, itself a mere postulate, we must add one 

 unknown to another, and assume that such birds 

 can inherit sensitivity to and recognition (not 

 necessarily mental) of particular components of the 

 magnetic field. This leaves us with a somewhat 



