1'KINCIPLES OF CLASSIFICATION. 29 



Mammals (as the Bats) being able to fly like Birds, whilst some 

 Birds are confined to the ground, like Mammals. 



6. From Reptiles, again, Birds differ in possessing a heart 

 with four cavities and a complete double circulation ; the heart 

 of Reptiles being furnished with only three cavities, and the 

 circulation being so arranged, that only a part of the blood sent 

 to the system has been exposed to the influence of air in the lungs: 

 and whilst the temperature of the bodies of Birds is kept up to 

 a certain high standard, that of the Reptiles varies with that of 

 the surrounding air, and is usually but little above it. There is 

 the same difference, too, in their mode of life, between Birds and 

 Reptiles, as between Birds and Mammals ; with this addition, 

 that Birds are the most active and energetic in their movements, 

 of all the vertebrated classes ; whilst Reptiles are the most inert 

 and sluggish. Between Birds and Fishes, the differences are 

 still greater ; for though they agree in being oviparous, they are 

 adapted for an opposite mode of respiration, the latter breathing 

 by water, whilst the former breathe by air ; the circulation of 

 Fishes, too, is much simpler in its plan, the heart having but two 

 cavities ; and the construction of their skeleton is such, as to 

 adapt these animals to rapid movement through a dense element, 

 water, in which they float without effort, instead of enabling 

 them to raise and sustain themselves in the comparatively unre- 

 sisting air. 



7. It appears, then, that a close affinity exists, between Birds 

 and Mammals on the one hand, and Birds and Reptiles on the 

 other. And we shall hereafter see, that this affinity is rendered 

 still more close, by the existence of certain species, in which the 

 characters of the respective classes to which they belong, are so 

 shaded off (as it were), and so blended with those of the neigh- 

 bouring classes, that we pass without any very abrupt break, 

 from one to the other. 



8. But it is not only in marking out these principal groups, 

 and in enabling us at once to know their points of general agree- 

 ment, as well as their most important differences, that classifica- 

 tion is useful. For the principal groups, or classes, are subdivided 

 into others, termed orders ; every one of which contains an 



