ENVIRONMENT, PAST AND PRESENT 81 



gatlon of problems of migration, but were a pigeon 

 not entirely non-migratory it could not be trained 

 to home. Only by virtue of its constant attach- 

 ment to one spot does it become the world's premier 

 homer. No less suitable subject could be selected. 

 When a young golden plover in its first fall leaves 

 the barrens it is doing exactly the opposite to homing 

 — it is deserting its home. 



In the homing of pigeons it seems certain that 

 sight and topographical memory are the salient 

 factors. We cannot here discuss the mass of evi- 

 dence but when critically analysed this conclusion 

 is inevitable. There seems little question that most 

 birds have wonderfully good memories while the 

 occurrence of two, or in some species, three foveae in 

 the retina of the eye can only mean one thing — ex- 

 ceptionally perfect vision. It is more than probable 

 that many species make use of these two faculties in 

 migrating. Of the many interesting facts brought 

 out by bird banding, none is more striking than the 

 return of given individuals to the same nesting box 

 year after year, in some cases with thousands of 

 miles of travel to their credit during the intervening 

 nine or ten months. That topographical memory 

 is involved in a feat of this nature seems more than 

 likely. But the return of an adult bird to its 

 nesting ground is homing in essence. It has been 



