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followed the courses of the latter in thek retreat before winter, 

 would have the best chance of maintaining themselves alive. 

 As the cold advanced from the north, birds will thus have early 

 acquired the habit of following coast lines trending to the south, 

 or some point approximating thereto, and those river valleys 

 whose course runs more or less in the same direction. It must 

 not be imagined, however, that without these guides, which 

 were, perhaps at most, merely guides to food, birds could 

 not have found their way, but that their presence in large 

 numbers in such locahties is due — as Herr Gatke himself points 

 out (p. 36) is the case at the present day — to actual necessity. 

 But what was once a necessity has since become a habit. It 

 may be urged against this theory that the height at which 

 many species now conduct their flights, is strongly against the 

 probability of the assumption that birds ever followed the courses 

 of coast lines and rivers. It may be admitted that the latter 

 have now ceased to be followed as guides. It is even doubtful 

 if this were ever the case except unconsciously so, but there 

 can be little doubt that the old routes of migration are still 

 adhered to simply from force of inherited habit. It is highly 

 probable that the movement at its earliest inception and perhaps 

 for a vast number of years afterwards was carried out at low 

 elevations and in daylight, after the manner of those supple- 

 mentary migrations occurring late in the winter, or those short 

 peregrinations to which Herr Gatke alludes on p. 46. The 

 habit of migrating by night was most probably acquired at a 

 later date and may have arisen from the fact that the daily 

 breezes both in spring and autumn, except in the case of very 

 strong winds and gales, are apt to die down at sunset, the 

 hours of darkness being characterised by calm. It is a curious 

 fact that the Hooded Crow — one of the regular day-fliers — is 

 declared by Herr Gatke to be more indifferent to the state of the 

 weather than all other migrants. 



So much has been said with regard to migrations over the 

 land. But the question arises — By what means did birds first 

 acquire the habit of crossing wide seas to winter quarters, of 

 whose very existence we have no reasons for thinking they were 

 cognisant? The subject is fraught with the greatest difficulty 

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