336 SYSTEMATIC ZOOLOGY 





open, and can run about and feed themselves ; such young birds 

 are called prczcoces or precocious. Others are nearly naked, 

 blind, require long feeding by the mother bird, — the male 

 rarely assists, — and have to be taught to fly. Such young are 

 called altrices, or non-precocious. There is usually considerable 

 sexual dimorphism in birds ; in most cases the male is larger 

 and stronger than the female, though sometimes smaller, and he 

 is usually more brilliantly colored and has a richer voice. Both 

 the coloration and the song are most brilliant at the breeding 

 season. 



Birds are found all over the world. Some live permanently 

 in the tropics, others in the temperate regions. As they never 

 hibernate, they cannot live permanently in excessively cold cli- 

 mates, and so they migrate in the spring from tropical to temper- 

 ate and even arctic regions, and in the autumn return to warmer 

 countries. The tropical species are bv far the most brilliantly 

 and beautifully colored. Fossils are rare in this class, but what 

 we have are of very great importance, since, as we have already 

 noted, they show a close relationship to some fossil reptiles, and 

 so we may infer a reptilian origin for the Aves. 



The group, because of the presence of feathers, is very sharply 

 marked off from all other groups of animals ; but the farther 

 classification of the many thousand species is a matter of the 

 greatest difficulty and one upon which scarcelv any two zoolo- 

 gists agree. For a long time birds were divided into two groups, 

 the Ratitae (Lat. raits, raft) and the Carinatae (Lat. carinatis, 

 keel-shaped), and this classification is even preserved by some 

 zoologists to-day. In the former group were placed the run- 

 ning birds, such as the ostrich, which have no keel on the 

 sternum, and in the latter all the birds which have a keel. The 

 Carinatas were then subdivided into a number of groups accord- 

 ing to the habits of the species, such as waders, scratchers, 

 swimmers, and birds of prey, a classification almost as unnatural 

 as it would be to place the marine serpents with the fishes be- 

 cause both inhabit the water. Consequently our revised classi- 

 fication rests on the structure and development of the different 

 species, to a considerable extent on the variations in the skull. 

 The following classification will serve as a practical foundation 

 for the student who wishes to make a more detailed study of 



