260 RELATION OF TERRITORY TO MIGRATION 



supplied the conditions under which, in the 

 process of time, this complex and definite mode 

 of behaviour has evolved. 



We are sometimes told that we must seek 

 the origin of migration in the physical changes 

 that have occurred in the ancient history of the 

 earth — in glacial conditions which gradually 

 forced birds to the south, or in the " stability of 

 the water and mobility of the land " which 

 brought about a gradual separation of the feed- 

 ing area from the breeding area — and which 

 continued for a sufficient length of time to 

 lead to the formation of an instinct, and that 

 the instinct persists because it is serviceable 

 in promoting the welfare of the race. But 

 when we consider the lapse of time, and the 

 changes that must have occurred in the character 

 of the bird population — the appearance of new 

 forms and the disappearance of the old, the ebb 

 and flow of a given species in a given area — and 

 bear in mind that, notwithstanding this, the 

 migratory instinct, if not stronger, is assuredly 

 no less strong, and the volume of migration, if 

 not greater, is assuredly no less ; in short, that 

 the whole phenomenon is progressive rather 

 than retrogressive, we shall find the view that 

 the instinct owes its origin to conditions which 

 no longer exist, receives but little encourage- 

 ment. 



I doubt not that, throughout the ages, 

 geological changes have been an important 

 factor in directing or limiting the scope of 

 migration, and moreover are so still ; just as 



