20 Introduction. 



are told in the history of their creation that they were formed 

 out of the water, and that they were made ' to fly above the earth 

 in the open firmament of heaven.' That, then, is their own 

 proper sphere, that the domain allotted to them to occupy. It 

 is true that we find some continually remaining in the element 

 from which they first derived their origin, passing almost all 

 their time in the water ; others again there are which seldom 

 leave the surface of the earth, and are neither formed for swim- 

 ming nor for flight ; but the great majority of species are essen- 

 tially denizens of the air, soaring high above our heads, skimming 

 here and there, floating with expanded wings, 'cleaving with 

 rapid pinions the vast aerial expanse.' Now it is clear that to 

 enable them to do this, the general formation of their bodies 

 must be extremely different from that of the Mammalia, though 

 to a certain extent there are strong resemblances and analogies 

 between them and their respective orders. As there are carnivo- 

 rous quadrupeds, so there are rapacious birds, and both are 

 equally fierce, sullen, unsociable, and solitary in their habits, 

 possessed of great strength, and often of considerable courage. 

 As there are herbivorous quadrupeds, so there are granivorous 

 birds, and both of these are gentle and gregarious in their habits, 

 a mild and tractable race, and easily domesticated. There are 

 also birds as well as beasts of an amphibious nature, having 

 organs suited to their habits, and these live chiefly in the water, 

 and feed on aquatic productions; and there are many similar 

 resemblances. Like the quadrupeds, too, they are warm-blooded 

 and vertebrate; but, unlike them, they are oviparous, and, 

 instead of fur, are usually clothed almost entirely with feathers ; 

 while instead of fore-feet they are furnished with wings ; and we 

 shall presently see that there are many other striking points of 

 difference in structure between them. Unlike the heavy bodies 

 of the Mammalia, which are formed to live on the surface of the 

 earth, the bodies of the birds are light and buoyant. They each 

 possess externally head, neck, body, tail, legs, and feet; but 

 instead of the large head, the heavy neck, the deep chest, the 

 wide shoulder, and the sinewy legs of the quadrupeds, the ob- 



