134 COMPARATIVE ANATOMY. 



can be occasionally puffed out, and its tendency to descend 

 in the air, or its specific gravity, made Jess. The bodies 

 of birds are blown up from their lungs, which no other ani- 

 mal bodies are; and thus rendered buoyant. 



VII. 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 burthen to a bird in its flight. The ad- 

 vantage in this respect, of an oviparous procreation is, that, 

 Avhilst the whole brood are hatched together, the eggs are 

 excluded singly, and at considerable intervals. Ten, fif- 

 teen, 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 egg at one time. 



VIII. A principal topic of comparison between animals, 

 is in their inMrnnients of motion. These come before us 

 under three divisions, feet, wings, and fins. I desire any 

 man to say, which of the three is best fitted for its use ; 

 or whether the same consummate art be not conspicuous 

 in them all. The constitution of the elements, in which 

 the motion is to be performed, is very different. The ani- 

 mal action must necessarily follow that constitution. The 

 Creator, therefore, if we might so speak, had to prepare for 

 different situations, for different difficulties : yet the pur- 

 pose is accomplished not less successfully, in one case 

 than in the other. And, as between icings and the corres- 

 ponding limbs of quadrupeds, it is accomplished without 

 deserting the general idea. The idea is modified, not de- 

 serted. Strip a wing of its feathers, and it bears no ob- 

 scure resemblance to the fore leg of a quadruped. The 

 articulations at the shoulder and the cubitus are much 

 alike ; and, what is a closer circumstance, in both cases 

 the upper part of the limb consists of a single bone, the 

 Sower part, of two. 



But, fitted up with its furniture of feathers and quills, it 

 becomes a wonderful instrument ; more artificial than its 

 first appearance indicates, though that be very striking : at 

 least, the use which the bird makes of its wings in flying, 

 is more complicated, and more curious than is generally 

 known. One thing is certain ; that, if the flapping of the 

 wings in flight were no more than the reciprocal motion of 

 the same surface in opposite directions, either upwards and 

 downwards, or estimated in any oblique line, the bird 

 would lose as much by one motion, as she gained by another. 

 The skylark could never ascend by such an action as this; 



