612 CHORD AT A. 



that both sexes build the nest, which with the weaver birds is most skil- 

 fully constructed; occasionally among social species the nests are placed 

 under a common roof. When the clutch of eggs is complete the female 

 (rarely the male) begins the incubation, at this time in some instances 

 losing the feathers from certain regions the better to warm the eggs. 

 Many birds, like hens and ducks, are so far advanced when they leave the 

 nest that they can follow the mother and feed themselves. Such birds 

 are called Prsecoces in contrast to the Altrices, which hatch with incomplete 

 coat of feathers and therefore need the warmth of the nest and the pro- 

 tection and care of the parents. 



The migrations of birds possess great interest. We distinguish among 

 birds permanent residents and others which, in order to obtain food, take 

 long journeys, the migratory species. At the approach of cold weather 

 these seek the south, following regular paths in their travels. They can- 

 not, like reptiles and amphibians, hibernate at the period when insects 

 and fruit are scarce, because their greater intelligence and their more ener- 

 getic vital processes demand a more rapid metabolism and a continuous 

 food supply. Hence the birds, like the mammals, in contrast to the 

 'cold-blooded ' reptiles, amphibia, and fishes, maintain, under all extremes 

 of external temperature, a body heat of 38-40 (44 ?) C. (100-104 F.). 



The classification of birds is in a state of change. The older system 

 based upon adaptive characters is not in harmony with the results of care- 

 ful anatomical study, which would divide the whole class into many small 

 groups. For this reason it has been thought best to retain the older sys- 

 tem of larger, easily recognized divisions, and to call attention, where 

 necessary, to the contradictions with later results. 



Order I. Saururae. 



The view that birds are closely related to reptiles has received 

 considerable support by the discovery of fossil birds with teeth. 

 The most reptilian of these occur in the Jurassic of Bavaria, and 

 only two specimens have been found. In these (Arch&opteryx 

 lithographica) the carpals and metacarpals have not fused, the 

 three fingers are well developed and clawed, and the caudal verte- 

 brae, although bearing feathers, form a long slender tail like that 

 of a lizard (fig. 2). 



Order II. Odontornithes. 



These forms, from the cretaceous of Kansas and Colorado, also 

 had teeth. In the ODONTOEM^: (Ichthyomis) there was a keeled 

 sternum and normal pygostyle. In the ODONTOHOLC^E (Hesper- 

 ornis) the wings were reduced (only the humerus persisting), the 

 sternum was without a keel, and the caudal vertebrae formed a 

 broad paddle. 



Order III. Ratitae. 



Here are included several families, very different in structure, 

 which agree in having the feathers not arranged in feather tracts;, 



