INTRODUCTION. Xlll 



birds remove from one country to another, or from the more 

 inland districts towards the shores : the times of these 

 migrations or flittings are observed with most astonishing 

 order and punctuality ; but the secrecy of their departure, and 

 the suddenness of their re-appearance, have involved the sub- 

 ject of migration in general in great difficulty. Much of this 

 difficulty arises from our not being able to account for their 

 means of subsistence during* the long flights of many of 

 those birds which are obliged to cross immense tracts of 

 water before they arrive at the places of their destination: 

 accustomed to measure distance by the speed of those ani- 

 mals with which we are well acquainted, we are apt to over- 

 look the superior velocity with which birds are carried for- 

 ward in the air, and the ease with which they continue 

 their exertions for a much longer time than can be done by 

 the strongest quadruped. 



Our swiftest horses are supposed to go at the rate of a 

 mile in somewhat less than two minutes; and we have one 

 instance on record of a horse being tried, which went at the 

 rate of nearly a mile in one minute, but that was only for 

 the small space of a second of time.* In this and similar 

 instances we find, that an uncommon degree of exertion is 

 attended with its usual consequences, debility, and a total 

 want of power to continue that exertion; but the case is 

 very different with birds; their motions are not impeded by 

 similar causes; they glide through the air with a quickness 

 superior to that of the swiftest quadruped, and they can con- 

 tinue on the wing with equal speed for a considerable length 

 of time. Now, if we can suppose a bird to go at the rate of 

 only half a mile in a minute, for the space of twenty-four 

 hours, it will have gone over, in that time, an extent of 

 more than seven hundred miles, which is sufficient to ac- 

 count for almost the longest migration; but if aided by a 

 favourable current of air, there is reason to suppose that the 



* See History of Quadrupeds. 



