112 SUBKINGDOM VEBTEBRATA. 



a curious problem. It is supposed that the upward pressure 

 of the air caught in the saucer-like hollow of the wing can- 

 not bend the inflexible bone and muscles of the front margin, 

 but curves the ends of the elastic quills upward, pushing 

 them forward, and so rorcing the bird ahead. 



Respiration takes place not only in the lungs, but also in 

 the substance of the other organs ; the air penetrating into 

 the interior of the bones and feathers, sometimes even to the 

 toes. So complete is this second process that it is said a 

 bird will breathe through the end of a broken bone when the 

 windpipe is tied. 



Hatching birds' eggs requires a varying length of time, 

 according to the species. The temperature needed is 104 

 Fahr. The chick first pierces its shell by a pegging motion 

 of the hardened point of the under mandible, and then 

 enlarges the aperture by pressing with the knob-like* end of 

 the upper mandible. Finally, by turning from left to right, 

 it breaks the shell about half around, when a vigorous stretch 

 of the body, assisted by bracing with the feet, parts the shell 

 transversely and the young bird is liberated. f 



The class of birds is divided into orders as follows :J 



* This falls off soon after the chick emerges from its shell, as Nature abhors a 

 useless appendage. 



t Sometimes the fracture extends entirely around the egg, and the two portions 

 are completely separated from each other. But if the inner membrane of the shell be 

 not wholly divided, the connecting portion serves as a hinge, and the two parts of 

 the shell may, in the movements of the young bird, become set, like two cups, one 

 within the other. 



$ The classification of Birds is unsettled. Ornithologists have not as yet agreed 

 even upon the number of orders. Attempts have been made to establish an ordinal 

 system. Thus the form of the bill has been made a b-isis of separating the almost 

 interminable series of Passerine birds into Conirostres^ with stout, conical bill; 

 Dentirostres, with a toothed and usually more or less hooked tip ; Tenuirostres, with an 

 elongated and awl-shaped bill ; and Fissiroslres, with a depressed, wide-gaped bilL 

 These distinctions have proved unreliable and are generally discarded. In such 

 uncertainty the classification here adopted is essentially that of Lilljeborg in " A 

 History of North American Birds," by Baird, Brewer, and Ridgway. It is, however, 

 considered only provisional. The fifteen orders given in the table have been sub- 

 divided by recent authorities into over one hundred families, several hundred genera, 

 and at least ten thousand species. The limits of this book will permit the descrip- 

 tion of only one or two species typical of each family selected, American examples 

 being generally chosen. 



