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" The formation of these conditions in the autumn usually 

 follows the passing away from Scandinavia — the area in which 

 the movement has its origin — of a spell of a more or less pro- 

 nounced cyclonic nature, during the prevalence of which the 

 ordinary course of the migratory movements is either interrupted 

 or rendered impossible. The effects of this sequence of meteoro- 

 logical conditions on bird migration are remarkable. 



" During the cyclonic spell a weather barrier arrests the 

 progress of, and dams back, as it were, the ordinary seasonal 

 migratory stream. These periods, too, are not unfrequently 

 characterised by weather of great ungeniality, and this, no 

 doubt, gives the summer birds warning that the time for seeking 

 the south has arrived. Upon the duration and severity of these 

 preliminary conditions depends, to some extent, the magnitude of 

 the migratory movement that follows. 



"The formation of the anti-cyclone removes the cyclonic 

 weather barrier, releases the flood, and provides conditions 

 favourable for migration, adding also an incentive in the form of 

 a decided fall in temperature." 



Mr. Clarke then further illustrates how birds, starting under 

 favourable meteorological conditions, on nearing Great Britain 

 may fly into conditions the reverse of favourable, owing to the too 

 close proximity or the depth of the western low-pressure centre. 

 He further remarks on the exhausted condition in which many 

 birds arrive under these circumstances, also stating that no 

 doubt many sometimes perish during the journey. It will be 

 seen that Mr. Clarke's conclusions are quite at variance with 

 those of Herr Gatke, for in the light of the theories of the latter, 

 migrating birds, on encountering the low-pressure system and its 

 accompanying unfavourable winds, would not have their flight 

 retarded or altogether prevented, but would merely elevate its 

 path to regions above the disturbances and pass over the shores 

 of Great Britain at heights beyond our cognizance. On the 

 other hand, he confirms Herr Gatke's statement that the 

 prevalence of easterly winds is favourable to the migratory flight, 

 but not in the same sense the latter asserts is the case. It has 

 been previously pointed out that Mr. Clarke's labours have 

 resulted in the opinion that Heligoland and Britain draw their 



