X. INTRODUCTION. 



stocked with a numerous set of inhabitants of the second 

 class the swimmers. Some of these, likewise, after having 

 reared their young, migrate much in the same way as the 

 waders. 



The ornithologist, who does not content himself with bare 

 names and appearance, in examining the economy of the 

 various kinds of birds, and the structure of their several parts, 

 will find ample room for the exercise of his labours in the 

 most minute investigation; and although he can scarcely 

 overlook the slow, and almost imperceptible degrees, by 

 which nature has removed one class of beings from another, 

 yet in his attempts to trace the relationship, or affinity, 

 which one bears to another, he will, with his utmost care, 

 find himself at a loss to ascertain that precise link in the 

 chain, where the doubtful crossing line is drawn, and by 

 which the various genera and species are to be separated. 

 But, however, after he shall have examined a few gradations, 

 upwards or downwards, he will more readily discover the 

 modes of life which the several kinds are destined to pursue; 

 and their ability to perform the various evolutions necessary 

 for the procuring of their food, in that exactitude to which 

 the Author of Nature hath formed them. In some of those 

 which run on the surface of the soft mud, and can occasion- 

 ally take the water, the indications of their ability for 

 swimming are furnished very sparingly: these indications 

 first appear in the breadth of the under sides of the toes, 

 with the two outer toes joined by a small web. The scal- 

 loped membranes attached to the sides of the toes form the 

 next advance: some are webbed to the nails, with deep 

 indentations in the middle, between each toe; others have 

 only three toes, all placed forwards, and fully united by 

 webbed membranes: some have the addition of back toes, 

 either plain, or with webbed appendages to each; and others 

 again have the four toes fully webbed together. The legs, 

 in the most expert divers, are placed very far back; they are 

 almost as flat and thin as a knife; and they are enabled to 



