XXX INTRODUCTION. 



expanded into fins, which, with the tail, are the organs of 

 progression. By contracting or expanding the air-sac, they 

 are enabled to alter the specific gravity of their bodies, and 

 rise or sink in the liquid in which they float. Their brain is 

 small ; their blood is red, but cold ; and the temperature of 

 their bodies is little above that of the surrounding element. 

 They are exceedingly voracious, preying incessantly the 

 strong upon the weak. Like all the other creatures, they 

 pass progressively from the simpler to the more developed, 

 until they are connected with the group above them, namely, 

 the Reptiles. 



The division Reptilia comprehends creatures varying greatly 

 in their forms, but all conforming to a common type. Some, 

 like fishes, have gills in the young state, the lungs being 

 only developed when they are able to quit the liquid me- 

 dium in which they are born, while a few retain both gills 

 and lungs through life, so that they are true Amphibia. 

 This group comprehends the Batrachian reptiles, the frogs, 

 the toads, the salamanders, and others ; the Chelonian rep- 

 tiles, as the tortoises and turtles ; the Saurian tribes, as 

 the lizard and crocodile ; and the Ophidean, comprehending 

 the snakes and serpents of all kinds. All the reptiles are 

 cold-blooded, and have a languid circulation. A few have 

 wings, and in a former age of the world the winged reptiles 

 were numerous, and of huge dimensions. The serpents, 

 partly aquatic, and partly living on land, are without feet, 

 and those which are inhabitants of land crawl upon the 

 ground, and many of them are furnished with a poison, with 

 which they are enabled to inflict deadly wounds. This sub- 

 stance, secreted by glands situated beneath the eyes, is con- 

 veyed to large tubular teeth in the mouth, by which the 

 venom is conveyed to the wound. 



Rising higher in the scale of organization are the beauti- 

 ful and varied tribes of Birds. The bodies of these creatures 

 are protected by light plumage : their posterior extremities 

 are limbs of support when at rest, and instruments of pre- 



