26 THE RIDDLE OF MIGRATION 



and returned a year or even two years later to A, 

 will unfailingly use the food and drinking appliances 

 and roosts in a way that leaves no doubt that they 

 remember details of their earlier home. The return 

 of wild birds to nest in the exact tree they used the 

 previous year, or to the same nesting box, indicates 

 a remarkable topographical memory. 



The important sense organs are found on the 

 head. This is to be expected since this is the end 

 of the animal that comes first into touch with its 

 environment. They make direct connections with 

 the brain, the centres involved tending to become 

 more and more elaborated and intercorrelated as the 

 sense organs themselves improve. With its increase 

 in size and complexity adequate protection becomes 

 more imperative and hand in hand with the evolu- 

 tion of the brain goes the development of the skull, a 

 protective contrivance. In the lowest of the verte- 

 brates it remains cartilaginous throughout life but 

 in the higher forms cartilage is replaced by the 

 harder material, bone, the skull being cartilaginous 

 only during a transitory embryonic period. The 

 bird's skull is bony but it has had to adapt itself 

 not only to the requirements of a brain and sense 

 organs but also to the carrying of materials, nest 

 building, climbing, etc. In other words, in large 

 part it has had to provide a substitute for the fore- 



