190 THE MIGRATION OF BIRDS 



spring, no one knew how or whence. During the 

 summer they were remarkable for being almost con- 

 stantly on the wing, but late in the autumn 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 swallow 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 migrating. 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 of 

 migration. The periodical disappearance of the swallow 

 remained, however, almost as great a mystery as before. 

 The impulse of migration was called an instinct, but did 

 nbt the less remain an unfathomable secret of Nature, 

 and the only cause that could be assigned for it was that 

 it must have been originally implanted in certain species 

 at their creation and denied to others. 



