8 COMPARISONS OF STRUCTURE IN ANIMALS, 



They rapidly traverse the air on wings, but 

 they repose on the ground, in the trees, or on 

 the surface of the water ; hence birds are 

 structurally modified for very diverse modes of 

 life. Quadrupeds produce living young, but 

 bii-ds lay eggs, upon which the parent incubates, 

 in order that the warmth communicated by her 

 body may cause the development of the chick. 

 Both quadrupeds and birds have warm red 

 blood. 



As we descend the scale, we come to groups 

 in which the blood is cold, and the body is 

 defended either with bony plates or shields, 

 with a sort of tesselated horny armour, with 

 scales, or is destitute of any covering. Some of 

 these animals have four limbs, some two, some 

 none. JMany are aquatic in their habits, others 

 terrestrial, and others arboreal. The greater 

 number breathe air ; some are constructed for 

 both aquatic and atmospheric respiration. 

 Some, during the first stages of their existence, 

 are fitted for aquatic respiration exclusively; 

 but, afterwards, from a strange alteration of 

 structure, lose the power of aquatic respiration, 

 and breathe only air. We allude to reptiles 

 and amphibious creatures — to tortoises, croco- 

 diles, lizards, snakes, sirens, and frogs ; animals 



