58 THE MIGRATION OF BIRDS. 



consequent supply of food, together with gradual 

 extension of range longitudinally, not essentially 

 dependent on climate, and that these movements are 

 the most recent in geological time. Second, that 

 moderate and long migration flight are the result 

 of a gradual extension of summer range, chiefly 

 latitudinally, owing to modification of climate, with 

 consequent increase of individuals, leading to an 

 expanse of winter area, and are of much older 

 initiation. Third, that extended migration flight 

 implies a complete change of breeding-ground, 

 more or less Inter-Polar, and is decidedly the most 

 ancient of all. 



In the present chapter we have glanced at the 

 probable origin and descent of Birds ; at the 

 changes of climate they have experienced from the 

 date of their evolution from a lower form ; en- 

 deavoured to show how these changes have been 

 brought about, and what the result has been. We 

 have glanced at some of the most powerful causes 

 of Migration, and traced the wonderful gradations 

 of Migration Flight to their origin both in their 

 mildest and in their acutest phases. We will now 

 conclude by following in detail the migration of 

 some single species, say from its Post-Pliocene 

 glacial initiation to the present day, in order clearly 

 to demonstrate Why the habit has been acquired, 

 and How it is practised. 



We will select the Spotted Flycatcher [Muscicapa 

 gri.wla) for the purpose. It is one of our best 

 known summer migrants, and one whose present 



