MIGRATION. 297 



pable of carrying them over a very large space in a 

 short time. The flight of birds generally may be 

 estimated at from fifty to one hundred and twenty 

 miles an hour ; and if we take the mean of this, we 

 shall find it sufficient to enable the migratory birds 

 to perform the most extended journeys. The won- 

 der is not in the flight itself, but in the impulse and 

 instinct by which it is commenced and carried on. 



Pennant finds no difficulty in accounting for the 

 motive of migrations : a defect of food at certain 

 seasons, or the want of a secure asylum from the 

 persecutions of man during the time of courtship, 

 incubation, and nutrition. He considers that most 

 of the birds which leave England in spring, to spend 

 the summer elsewhere, have been traced to Lap- 

 land, a country of lakes, rivers, swamps, and alps, 

 covered with thick and gloomy forests, that afford 

 shelter during summer to these fowls, which in 

 winter disperse over the greater part of Europe. 

 In these arctic regions, in consequence of the 

 thickness of the woods, the ground remains soft 

 and penetrable to the woodcocks and other slen- 

 der-billed fowls ; and for the web-footed birds the 

 water affords innumerable larvae of the gnat. The 

 days are there long, and the beautiful meteorous 

 nights indulge them with every opportunity of 

 collecting so minute a food, while mankind is 

 very sparingly scattered over those vast northern 

 wastes. 



The migration of winter birds of passage doubt- 

 less proceeds on the same general law as that which 

 regulates the movements of those birds which spend 

 the summer in England and leave it in winter. 

 Birds which find the temperature and circumstances 

 of summer in that country most congenial to their 

 wants and habits, retire on the approach of. severe 

 weather to find something similar in the south; 

 while others, which remain there in winter to 

 avoid the extreme rigour of that season in the most 



