CHAPTER XI 



SUMMARY 



Migration owes its origin to the potentiality of 

 flight, enabling birds to advantage themselves by 

 extended dispersals, which through heredity be- 

 come instinctive, regular and periodical. Geo- 

 logical changes, especially the passing away of the 

 glacial epoch, only influenced by opening up new 

 lands for summer colonisation, but climatic con- 

 ditions prevented these lands from becoming per- 

 manent abodes and fostered the habit of periodical 

 migration. Whatever the original home or centre 

 of distribution may have been, the dispersal from 

 it was towards new lands with a retreat towards 

 the food-supply when these lands became untenable. 

 Fluctuating food - supply, love of home, sexual 

 impulses, desire for light, varying temperature, 

 and other factors, all have more or less influence, 

 but the force exerted by any or all depends upon 

 the species operated upon and the locality in which 

 it resides. The present route followed or method 

 of migration is little guide to the history of past 

 migration ; during the evolution of present-day 

 migration alterations may have been occasioned by 



126 



