164 THE STORY OF THE BIRDS 



Swallows and Hudsonian Godwits, with a journey 

 of from 7000 to 10,000 miles. These extreme 

 limits, however, only represent the length of the 

 migration of the species, not of the individual, for 

 in many cases we have two sets of individuals of 

 the same species, one set moving north, the other 

 set moving south, from the equatorial centre of 

 dispersal. 



A few words now on what we may term the 

 philosophy of migration. We have briefly dealt 

 with what we believe to be the Cause of migra- 

 tion ; now let us try and ascertain how and 

 under what conditions the wondrous journey 

 is accomplished. In a preceding chapter where 

 we had the various bird groups under review, 

 attention was repeatedly drawn to the fact that 

 many families were remarkable for their long 

 pointed wings. It is in such groups as these 

 that we find the birds most remarkable for their 

 migrations. Generally speaking, a bird must 

 have wings adapted to quick and sustained 

 flight to be able to migrate successfully ; hence 

 the rule is for birds of passage to be long 

 winged and capable fliers. These long, power- 

 ful wings have been developed by successive 

 generations of regular migration flight : the 

 birds that fly most have the highest developed 



