272 RELATION OF TERRITORY TO MIGRATION 



Now certain facts are presented to observa- 

 tion which enable us not only to understand the 

 nature of the change that is wrought in the 

 history of the species, but to foreshadow, with 

 no small degree of certainty, the extent of that 

 change. 1 suppose that it has come within the 

 experience of most of us to observe, at one time 

 or another, the ebb and flow of a given species 

 in a given district. Some favourite haunt is 

 deserted for a year, or for a term of years, and 

 is then revisited ; or, if it is always occupied, 

 the number of inhabitants fluctuates — plenty 

 of pairs in this season, only a few in that. 

 Many intricate relationships, both external and 

 internal, contribute towards this state of affairs. 

 Fluctuation in a downward direction, or 

 temporary extinction, is brought about by 

 changes in the physical world, by changes in 

 the available supply of food, by the increase 

 of enemies, or by adverse climatic conditions ; 

 whilst fluctuation in an upward direction, though 

 due indirectly to a combination of circumstances 

 in the external world favourable to the survival 

 of large numbers of individuals, is directly 

 determined by the impulse to seek isolation. 

 As individuals of diff^erent species establish 

 themselves, and form kingdoms and lesser 

 kingdoms, we can watch the gradual quickening 

 into life of moorland and forest and we can 

 observe the manner in which it all comes to 

 pass. Males that for weeks or months have 

 lived in society, drifting from locality to locality 

 according to the abundance of food or its 



