COMPARATIVE ANATOMY. 155 



of quadrupeds ; secondly these cavities are empty ; thirdly, 

 the shell is of a firmer texture than is the substance of other 

 bones. It is easy to observe these particulars even in pick- 

 ing the wing or leg of a chicken. Now, the weight being 

 the same, the diameter, it is evident, will be greater in a 

 hollow bone than in a solid one ; and with the diameter, as; 

 every mathematician can prove, is increased, cceteris paribus, 

 the strength of the cylinder, or its resistance to breaking. In 

 a word, a bone of the sa7ne iveight would not have been so 

 strong in any 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 

 tabular by exercise. What appetency could excavate a 

 bone ? 



VI. The lungs also of birds, as compared with the lungs 

 of quadrupeds, contain in them a provision distinguishingly 

 calculated for this same purpose of levitation, namely, a 

 communication — not found in other kinds of animals — be- 

 tween 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 pufied out, and its tendency to 

 descend in the air, or its specific gravity, made less. The 

 bodies of birds are blown up from their lungs — which no 

 other animal bodies are — and thus rendered buoyant. 



YII. All birds are oviparous. This likewise carries on 

 the work of gestation with as little increase as possible of the 

 weight of the body. A gravid uterus would have been a 

 troublesome burden to a bird in its flight. The advantage 

 in this respect of an oviparous procreation is, that while the 

 whole brood are hatched together, the eggs are excluded 

 singly, and at considerable intervals. Ten, fifteen, or twenty 

 young birds may be produced in one cletch or covey, yet the 

 parent bird have never been encumbered by the load of 

 more than one full-grown Qgg at one time. 



VIII. A principal +opic of comparison betw^een animals, 



