28 PRINCIPLES OP 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. . V 



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 nurnber 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 Vertebrated 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 wiih others, which are ordinarily and most conveniently 

 designated by English names. 



