46 DEFINITION AND 



ent on its external appearance, is so simple and so 

 well understood, that the repetition of it would be 

 superfluous. It is a vertebrated animal ; and, be the 

 species what it might, no one could mistake it for one 

 of the mammalia, for a reptile, or for a fish. There 

 have been some mistakes the other way, though they 

 have been but few : in the infancy of natural science 

 bats were considered as a sort of birds : and some of 

 the moderns, who have not had opportunities of 

 studying the physiology of the animal, have made 

 a sort of bird of the ornithorhynchus. 



These mistakes show, what we find to be the fact 

 when we make the attempt, that the natural, or even 

 the satisfactory classification of birds according to an 

 artificial system, is no easy matter. The bats were 

 called birds on account of their flying membranes, 

 and the ornithorhynchus because of its mandibles, 

 which are something (but not very) like those of a 

 duck's bill, while both animals had all the essential 

 characters of true mammalia, though mammalia of 

 peculiar form and habits. Thus it appears that 

 neither the bill nor the flight of birds can be taken 

 as the ground of a classification ; as little can the 

 feet ; and the digestive organs merely point out the 

 general kind of food, and not how or where it is 

 obtained. 



The feeding of birds cannot be made so good a 

 means of general distinction as that of the mammalia, 

 because many birds are so very miscellaneous in what 

 they eat that no one article can be considered as their 

 characteristic or leading food. And when we take 

 the three leading characters : the bill and digestive 

 organs as the system of nourishment, the wings as the 

 organs of motion in the air, and the feet as organs of 

 motion, we find that not even any two of them vary 



