CHAP. XX. HYBERNATION. 243 



there is no evidence of the hybernation of any species of 

 birds. Never was theory founded on more flimsy substratum 

 of supposed facts, or supported by weaker logica*argument. 

 Swallows made their appearance in spring, no one knew how 

 or whence. During the summer they were remarkable for 

 being almost constantly on the wing, but late in the au- 

 tumn a noticeable change took place in their habits. They 

 were observed more and more to congregate in large flocks, 

 and towards evening to perch in numbers on houses, and 

 frequently in trees. Finally they collected in thousands 

 on the reeds and willows on the banks of the Thames, and 

 disappeared during the night. In the morning not a swal- 

 low was to be seen, and ornithologists came to the startling 

 conclusion that they had plunged into the dark waters of 

 the river, and buried themselves in the mud at its bottom, 

 to reappear the following spring, refreshed by their winter's 

 sleep. And this theory was gravely accepted as one among 

 many other unfathomable secrets of Nature ! After a time 

 it was, however, discovered that the disappearance of many 

 of the summer resident birds from their breeding-grounds 

 in this country was the signal for their appearance in some 

 parts of Southern Europe or Northern Africa, and the 

 theory of migration was accepted as resting upon a basis 

 of indisputable fact when birds were seen in the act of mi- 

 grating. At certain stations, such as Gibraltar and Malta, 

 and notably on the island of Heligoland (all, curiously 

 enough, under British rule), birds were seen passing over, not 

 in small flocks only, but by thousands and tens of thousands, 

 so that no possible doubt could remain as to the great fact 



