DISPERSAL AND MIGRATIONS 153 



a more or less equatorial base, all the early pro- 

 genitors of that species may have from a variety 

 of causes absolutely disappeared, and left their 

 descendants as colonists in new countries far 

 remote from their primal origin, without a trace 

 being left to indicate the line of their dispersal. 

 In some cases we can form a pretty accurate 

 idea of the general route by the various allied 

 and representative species which have estab- 

 lished themselves nearer to the point of dis- 

 persal. With many migratory birds, however, 

 the ancient centre of dispersal, or the range 

 base from which the species commenced its 

 colonising movement, is still returned to and 

 used as a winter home. Here we may also state 

 that the geographical area of nearly every species 

 is continuous. Broken areas almost invariably 

 lead to specific differences. There are one or 

 two instances of apparently discontinued area 

 (Asio capensis for example), the species remain- 

 ing the same in each locality. But this may 

 be due to a recent interruption, and eventually 

 result in specific segregation should it remain 

 permanent. Many species may possibly have 

 originated on various islands as some large land 

 mass became split up into an archipelago by 

 volcanic or other action. As a rule, the more 



