28 PRINCIPLES OF CLASSIFICATION. 



constructed, there is a remarkable correspondence between several 

 parts of their respective fabrics. Hence, they are termed analo- 

 gous groups ; and may be said to represent one another in the 

 series to which they respectively belong. Thus, Birds may be 

 termed the Insects of the Vertebrated series ; and Insects may 

 be regarded as the Birds of the Articulated. We shall have 

 several occasions for noticing similar analogies, in the course of 

 this Treatise. 



5. But we shall next inquire, how the characters which have 

 been mentioned as distinguishing the class of Birds, serve to sepa- 

 rate it from others, which have more in common with it. The 

 classes of Reptiles and Mammals * all agree with Birds in a 

 larger number of characters, and these, too, of the highest 

 importance, than do Insects ; notwithstanding that the differ- 

 ence in external form, and in habit of life, may be much greater. 

 For they are all Yertebrated animals, possessing a jointed internal 

 skeleton, which envelopes and protects the nervous centres, a 

 distinct brain and spinal marrow, four members or extremities, 

 five senses, red blood, simple eyes, &c., &c. With Mammalia 

 and Reptiles, Birds further agree, in breathing air by means of 

 lungs ; whilst they agree with Reptiles and Fishes in being 

 oviparous. With Mammals, again, they agree in having a heart 

 with four cavities, and a complete double circulation (which is so 

 arranged, as to expose all the blood to the air in the lungs, before 

 it is again transmitted to the system) ; and also in constantly 

 maintaining a high temperature. But they differ from Mammals 

 in being oviparous instead of viviparous ; that is, in depositing 

 eggs, instead of producing their young alive, and nourishing them 

 afterwards by suckling ; and in having their bodies covered with 

 feathers, instead of with hair or scales. They differ, too, in the 

 mode of life for which they are adapted ; the Mammals (as a 

 class) being destined to live on the ground, whilst the Birds 

 inhabit the air : but there are exceptions in both instances, some 



* This modification of the scientific term Mammalia, having been sanctioned 

 by high authority, will be used in this work ; when it is requisite to speak of the 

 class in connection with others, which are ordinarily and most conveniently 

 designated by English names. 



