EARLY IDEAS OF MIGRATION 115 



and few early travellers were philosophers, at any 

 rate so far as migration was concerned. In Germany, 

 however, the Emperor Frederic II. realised in the 

 thirteenth century many truths concerning migra- 

 tion (27), but in Britain uncertainty or myth held 

 sway until the end of the eighteenth century. Herr 

 Herman, reviewing the variation in thought, says 

 " But as in other fields, this period is followed 

 by a time of decadence, a natural consequence of 

 departing from immediate experience." 



British, and many Continental observers too, 

 saw when birds had come and in autumn that they 

 had gone. Early swallows and martins were always 

 met with near water, and were watched dropping 

 to roost in the reed beds, as they always do in 

 autumn before departure. Next morning none 

 was visible. Certainly then they had vanished 

 to hibernate in the water. The discovery of masses 

 of torpid swallows, dead or dying, by no means an 

 unknown thing when birds are overtaken by sudden 

 falls in temperature in autumn or by a severe set- 

 back in the spring, was to these puzzled men 

 confirmation of their theory of hibernation. Other 

 details of the many stories of swallow hibernation 

 are due to exaggeration or to misconception. In 

 the second half of the eighteenth century a fierce 

 discussion waged for and against hibernation, and 

 many, including Geoffroy St Hilaire and Montagu, 



