32 



What is a Bird? 



features, birds are distinctly allied with rep- 

 tiles, so that in lieu of the former division 

 of vertebrates into four or five great pri- 

 mary divisions, or classes, namely, mam- 

 mals, birds, reptiles, batrachians and fishes, 

 modern zoologists divide them into three 

 primary groups, the mammals forming one. 

 Mammalia, the birds and true reptiles an- 

 other, called Sauropsida (or reptile-like ani- 

 mals), while the batrachians and fishes form 

 the third, called Ichthyopsida (or fish-like 

 animals). The connecting links between 

 birds and reptiles are furnished,. not by the 

 living forms, but by extinct forms of both 

 birds and reptiles, the early types of birds 

 presenting many reptilian features not shown 

 by any living birds, while some of the ex- 

 tinct reptiles were more bird-like than any 

 now existing. 



Less than a generation ago, birds were 

 thought to be one of the most fundamen- 

 tally distinct and most trenchantly defined 

 of the classes of vertebrates ; but recent 

 discoveries of extinct types have largely 

 broken down the then supposed sharp bar- 

 riers of distinction. 



There is still left, however, one charac- 

 teristic, though at first sight trivial and 

 superficial, which distinguishes birds from 

 all other animals, namely, the clothing of 

 birds. All birds have feathers, and no other 

 animals have them. Even the earliest known 

 extinct forms of birds wore feathers. Al- 

 though some feathers are hair-like in ap- 

 pearance, as the bristly plumes about the 

 beak in many birds; and while they are 

 sometimes half scale-like in form, as those 

 on the wing of a penguin, they are radically 

 distinct from either hair or scales in their 

 origin and character. Feathers, like hair 

 and scales, are appendages of the skin, and, 

 like them, are tegumentary, or organs of 

 covering. Jkit recent investigations tend 

 to show that they differ radically in their 

 method of growth, or in origin. Hairs are 

 formed by a solid ingrowth of the epidermis, 

 or superficial layer of the skin; feathers and 



scales are an outgrowth from a papilla. 

 Birds, like reptiles, are also provided with 

 scales or scutes, the feet (tarsi and toes) 

 in most birds being covered with true scales. 

 While the real difference as to their method 

 of origin has not been clearly shown, it has 

 been assumed that because feathers some- 

 times grow upon scales they must be differ- 

 ent in origin. 



The feathers of birds not only serve the 

 function of clothing, as hair in mammals, 

 and as scales in reptiles and fishes, but also 

 as an important accessory to locomotion, 

 for birds fly by means of long, stiff, strong 

 quills attached to the bones of the wing, 

 and the similar feathers forming the tail.* 



A bird being preeminently a flying verte- 

 brate, its whole structure is modified to 

 that end. The body itself has somewhat 

 the form of a double cone, of which the 

 neck and head, with its pointed beak, form 

 the anterior, and the tail the posterior apex. 

 This gives a form well adapted to easy 

 passage through the air. The covering of 

 feathers, closely overlapping and directed 

 backward, combines lightness with warmth; 

 the feathers themselves, while presenting a 

 more or less firm and smooth exterior, form 

 a non-conducting medium admirably adapt- 

 ed to prevent the escape of heat from the 

 body, the interior downy portion of the 

 feathers having their meshes filled with air. 

 The bones of the wing are lengthened to 

 give attachment to the flight feathers, the 

 length of the bones, and the firmness and 

 length of the feathers attached to them 

 varying with the power of flight in differ- 

 ent birds, according to their habits. Thus 

 in the birds of prey, the swallows, swifts . 

 and hummingbirds, in the terns, gulls, pe- 1 



♦ Bats only among mammals, or indeed among existing ver- 

 tebrates, possess true flight ; but a bat's wing is very unlike 

 the wing of a bird, the means of support being the greatly 

 lengthened fingers, covered with a leathery membrane, .to 

 which the attenuated fingers serve as a framework. The 

 ancient flying reptiles, known as Pterodactyles, had wings 

 apparently much like those of bats, and thus wholly unlike 

 those of birds, in which lengthened feathers, instead of mem- 

 brane stretched on a bony framework, form the means of 

 aerial locomotion. 



