INTERNAL MIGRATIONS 279 



place in a Northern Glacial Epoch — that utterly fictitious 

 southern emigration of life — they are purely local, and 

 can never extend beyond the geographical limits of the 

 species partaking in them. The Law of their dispersal, 

 which forbids sojitJiern extensioft of range eitJier in summer 

 or winter, and only permits northern extension or emigration 

 at all during or Just prior to the season of reproduction, 

 is inexorable and immutable. 



Having thus placed the subject on what I believe to 

 be a thoroughly sound and satisfactory basis, we will 

 pass to a more detailed study of the phenomenon. If 

 we admit that the whole phenomenon of Local Movement 

 or Internal Migration is confined within certain limits 

 and controlled by law to those limits, I think we must 

 also admit that the individuals partaking in it conform to 

 certain governing impulses. We have seen how closely 

 species and individuals and their descendants are con- 

 fined to certain routes, to certain limits, from which, 

 broadly speaking, they seldom diverge. It is therefore 

 difficult to believe that all this winter migration is 

 entirely fortuitous. Birds, individuals, have certain 

 routes, inhabit certain areas or districts to which they are 

 attached. It seems probable, then, that the individuals 

 partaking in these various winter migrations are moving 

 to and fro, according to changes of weather in the area 

 they occupy, along certain routes, which routes indicate 

 the lines of past emigration or range expansion. Birds 

 move south or north along a coast, or east and west 

 across a sea, but there is a certain amount of method in 

 the flight. Now most of this winter migration, across the 

 North Sea for instance, takes place within a compara- 

 tively narrow area. No matter how vast may be the 



