S40 NATURAL THEOLOGY. 



Other form ; and to have made it heavier would 

 have incommoded the animal's flight. Yet this 

 form could not be acquired by use, or the bone 

 become hollow^ or tubular by exercise. What ap- 

 petency could excavate a bone ? 



VI. The lungs also of birds, as compared with 

 the lungs of quadrupeds, contain in them a provi- 

 sion distinguishingly calculated for this same pur- 

 pose of levitation, namely, a communication (not 

 found in other kinds "of animals,) between the air- 

 vessels of the lungs and the cavities of the body ; 

 so that, by the intromission of air from one to the^ 

 other, (at the will, as it should seem, of the animal,) 

 its body can be occasionally puffed out, and its 

 tendency to descend in the air, or its specific gra- 

 vity, made less. The bodies of birds are blown 

 up from their lungs, (which no other animal bodies 

 are,) and thus rendered buoyant.^^ 



VII. All birds are oviparous. This likewise 

 carries on the work of gestation with as little in- 

 crease as possible of the weight of the body. A 

 gravid uterus would have been a troublesome bur- 

 den to a bird in its flight. The advantage in this 

 respect of an oviparous procreation is, that whilst 

 the whole brood are hatched together, the eggs 

 are excluded singly, and at considerable intervals. 

 Ten, fifteen, or twenty young birds may be pro- 



** We have thrown some observations upon this subject into 

 the Appendix, under the title of " The Relation of the Bodies of 

 Birds to the Atmosphere." 



