109 



THE CAUSE OF THE MIGEATOKY MOVEMENT 

 AND WHAT GUIDES BIRDS DURING THEIR 

 MIGRATIONS. 



HOUGH Herr Gatke has devoted separate chapters 

 to the discussion of these two features of the 

 phenomena of migration, they may be conveniently 

 examined together, for it will be found that one, 

 not unnaturally, throws light on the other. 



At the outset it may be stated that in neither 

 case does he put forth any explanatory theories of 

 his own ; indeed, he frankly confesses his disinclination to even 

 enter upon such a hopeless task. 



The following paragraphs are an attempt to embody the views 

 which have found most general acceptance as to the "cause of 

 the migratory movement," supplemented by a few suggestions on 

 the part of the writer. 



The principal factor necessitating these long journeys on the 

 part of birds from their breeding grounds, to winter quarters in 

 distant lands, is undoubtedly lack of food ; cold of itself, would 

 probably not have sufficient direct influence on the majority of 

 species to induce them to leave the homes to which they seem so 

 devotedly attached. Nevertheless, the prime originating cause of 

 the habit was, undoubtedly, the setting in of rigorous conditions 

 in what was probably their ancestral home, thus necessitating its 

 temporary abandonment. A low temperature of any duration 

 accompanied by a heavy fall of snow, means, to the great 

 majority of birds, the cutting off of all food supplies. Geological 

 evidence is pretty conclusive that at one time those regions in 

 the north which now endure the severities of an arctic winter, 

 previously enjoyed a fairly equable climate, perhaps akin to that of 

 our own islands at the present day. At the former period these 



