20 LECTURES ON BIOLOGY 



Let us now turn to the birds and observe the flight of the 

 sea-gull. It skims over the surface of the water, now rises into 

 the air. circles round and round, or hangs in mid-air, motionless. 

 Suddenly it darts down, quick as lightning, to capture its prey 

 with unerring aim. All is lightness, elegance, power. Let us 

 see how the development of its extremities will demonstrate the 

 fitness of its organization and the transforming influence of 

 function. 



The first feature noticed in a comparison of the wing of 

 a bird with the forefoot, for instance, of a salamander or lizard 

 is the great elongation of the various skeleton-parts. Everything 

 seems to have been arranged with a view to obtaining a long 

 and strong lever to which to join the feathers. The more skilful 

 the flier, the more pronounced the lengthening of the bones of 

 the arm and hand. We also find that not only the carpal and 

 metacarpal bones but also those of the fingers have decreased 

 in number and that some have become fused together into 

 a uniform bone. This fusion caused a considerable lengthening 

 of the skeleton of the hand without affecting its firmness. The 

 skeleton of the Archaeopteryx shows that this bird was a very 

 awkward flyer, making only a faint attempt to conquer the air ; 

 it also proves its descent from lizard-like ancestors, for its hand 

 had three well developed free-moving fingers armed with sharp 

 claws which probably enabled it to climb rocks and trees. 



In spite of the wonderful adaptation of the wing of birds to 

 flying, numerous birds have felt themselves induced to return 

 to terra firma and to depend more upon their legs. The cause of 

 this action is obscure. But we find that a more frequent use 

 and consequently stronger development of the hind-organs entail 

 a degeneration of the wings and a corresponding decrease in the 

 power to fly. There is a complete chain, perfect in every link, 

 which leads from the Gallinaceous Birds to the Ostriches and 

 from them to the Kiwis. Whilst the Pteroclidae (Sand-Grouse) 

 are still able to fly well and move chiefly by means of their 

 wings, the Phasianidae (game-birds, fowls, &c.) occupy a half-way 

 position, for they use fore and hind extremities in about equal 

 proportions. The Tinamous (Tinamus), finally, greatly dislike 

 flying, and in case of danger prefer to trust to their legs. These 

 different degrees in the use of wings are indicated externally 

 by their decreasing size. 



