THE " FOUR-FOOTED " BIRD 



very faintly discounted by the fact of the " quadru- 

 pedal " movements of the young of the Hoatzin and a 

 few others. No birds have teeth ; that is, no living 

 birds, for a few now extinct forms had these structures. 

 Every bird has a horny beak, which in some sense stands 

 in lieu of teeth. It is noticeable that birds have an 

 erect and alert attitude, which contrasts with that of 

 the gloomy reptile. The eyes are clear and open, not 

 closed, except, indeed, during actual sleep, in a 

 drowsy fashion. Hearing as well as sight is well 

 developed, and, indeed, the only sense in which birds 

 are as a rule rather deficient is that of smell ; but some 

 experiments have gone far toward showing that after 

 all, birds can detect varying odours rather more accu- 

 rately than has been supposed. In external form birds 

 are most obviously built upon one common plan. This 

 is recognized in popular parlance by the fact that a 

 given bird is apt to be spoken of merely as a bird, while 

 a mammal is more usually classified roughly ; it is 

 spoken of as a cat, or an elephant, etc. The flight of 

 birds, as well as their lightness of movement when 

 hopping upon the ground or among trees, is a character- 

 istic of the group, and is associated with air spaces 

 which ramify among the organs of the body and in the 

 substance of the bones. These air spaces are outgrowths 

 of the lungs ; and while they assist in producing a light 

 frame suitable for flying, must also, one would imagine, 

 improve the respiration of the creatures by bringing air 

 into contact with the entire system. No living reptile 

 or mammal has anything approaching to this aeration 

 of the body. It is possible, however, that certain 

 extinct reptiles, particularly the flying Pterodactyles, 

 had something of the kind ; so too had, in the opinion of 

 most, the hopping and often gigantic Dinosaurs of the 

 past. Associated with, and perhaps due to, this is the 

 hotbloodedness of birds. Their temperature is higher 



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