127 



explanations accounting for the first mentioned fact have already 

 been discussed, and whether we accept the theory of an east-to- 

 west line of flight or not, matters very little, as it is undoubtedly 

 the case that the winters in Western Europe are milder than 

 those in the east. Moreover there is absolutely no evidence to 

 prove that the old birds set out so very long after their progeny ; 

 indeed, observations all tend to prove that the latter migrate 

 slowly at first and in short stages, and that in many cases it is 

 only the actual advent of wintry conditions that brings about 

 those rapid torrents of migration alluded to by Herr Gatke on 

 p. 146. Whether birds fly by daylight or in darkness has no 

 bearing whatever on this point, and while there may be no 

 Itemptation to break the flight at an inhospitable little island, the 

 reverse is probably the case on encountering more favourable 

 localities. 



Herr Gatke concludes his chapter on " What Guides Birds 

 during their Migration," with a reference to the rare occurrence 

 of a inigration which may be described as not only retarded, but 

 actually turned back, through meeting with wintry conditions in 

 the land towards which it was travelling. In his view this 

 occurrence was similar to that experienced by Seebohm in the 

 valley of the Yenesay, when the latter was awaiting the break 

 up of winter on the arctic circle. Another interpretation of the 

 phenomena, however, as regards this particular instance may be 

 put forward. Mention has frequently been made of those supple- 

 mentary migrations which so frequently take place at Heligoland 

 late in the winter. Birds suddenly appear travelling past the 

 island in large numbers, their presence at these times being said 

 to unfaiUngly indicate the proximity of severe weather. We are 

 unfortunately told but little of this interesting incident beyond 

 the fact that on becoming aware of the strong migration in pro- 

 gress, the author came to the conclusion that the spring move- 

 ment had set in with remarkable force and at an earUer period than 

 usual. On becoming aware, however, of the direction in which 

 the birds were travelling, after considerable reflection, he came to 

 the conclusion that the flocks comprising the flight had already 

 once passed the island, and were now compelled by the severe 

 weather encountered ahead to retrace their steps. May it not 



