6o THE MIGRATION OF BRITISH BIRDS 



Law, which the reader must repeatedly bear in mind in 

 deahng with the facts presented. 



Law of Dispersal. — Northern Hemisphere species 

 never increase their Breeding Area Southwards — always 

 North, East, or West from a Range Base or Centre of 

 Dispersal, which contained the sole surviving portion of 

 the species during glacial ages. Inter-polar and Inter- 

 hemisphere species north of the Equator never extend 

 their breeding range Southwards if suitable areas are 

 open to the North ; Inter-polar and Inter-hemisphere 

 species south of the Equator never extend their breed- 

 ing range Northwards if suitable areas are open to the 

 South. The tendency of Birds, if not of all living 

 organisms, is to spread in the direction of the Poles. 

 During present time a bird in the Northern Hemisphere 

 never increases its range in a Southern direction; it 

 may do so North, North-east or North-west, East or 

 West ; but that it never does so South is proved by 

 the fact that no Migration Route is known which trends 

 South in spring, or which trends North in autumn. 

 During present time a species in the Southern Hemi- 

 sphere never increases its range in a Northern direction ; 

 it may do so South, South-cast or South-west, East 

 or West, but that it never does so North is proved 

 by the fact that no Migration Route is known which 

 trends North in spring, or which trends South in 

 autumn. The route of Present Migration is always 

 an unerring guide to, and unfailing indication of, the 

 general line of Past Emigration. 



From this Law we draw the following Corollaries : 

 Corollary I. — Migratory birds winter at the present 

 time in the Glacial Refuge Areas or Range Bases 



