34 THE MIGRATION OF BIRDS. 



to the well-being of such a vast number of species 

 must have had its origin in a very remote past, the 

 result of mighty climatic change and physical dis- 

 turbance, seems not only absolutely certain, but the 

 only rational explanation of the phenomenon. Such a 

 cause amply sufficient in every respect is to be found 

 in varying phases of Earth's orbital eccentricity in 

 combination v^ith the precession of the equinoxes — 

 the grandest cycle of discovered Time, which in 

 wondrous course entirely reverses the seasons of 

 either hemisphere as the earth's polar axis describes 

 a complete circle in the heavens. That these 

 majestic phenomena are in any conceivable way 

 connected with the migratory movements of birds 

 seems utterly impossible; but in them the habit has 

 its root ; and the simple season-flight of a Cuckoo 

 or a Nightingale to and fro between the shores of 

 Africa and England is inseparably and directly con- 

 nected with the erratic movement of a planet in its 

 orbit ; nay, with the constitution of a universe ! 



Of course in the utter absence of any data to fix 

 the northern range of migratory birds in remote 

 ages, it is impossible to give detailed explanations of 

 the cause of the habit in each species, or even in 

 many groups. Many local influences may have 

 been at work ; the gradual extension of range in 

 various directions from a centre of dispersal owing 

 to the rapid multiplication of a species may have 

 driven many individuals into more northern regions 

 where only a summer residence was possible ; or 

 even caused longitudinal movements necessitating 



