THE “FOUR-FOOTED ” BIRD 
6 
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 leu 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 
152 
