"THE RISE OF LAND LIFE 35 



Their defeat or senescence, failure and practical disappear- 

 ance is still hard to understand. 



From some not very primitive reptile, birds are descended. 

 If swiftness of locomotion is the goal of animal development, it 

 culminated in birds. Coordinated with and stimulated by 

 their swift loaomotion is the high development of the sense- 

 organs in the head, the look-out of the flying-machine. The 

 eyes are enormous, the sense of smell is exceedingly keen; 

 song suggests well tuned ears. The body is compact and 

 light to the last degree. The light stiff skeleton is a marvel- 

 lous piece of engineering architecture. The back-bone, com- 

 pletely ossified, stiffens the body for flight. The bones of 

 the shoulder girdle are admirably modelled and arranged to 

 meet the strain of the heavy muscles which produce the down- 

 stroke of the wings. The head is lightened by replacing 

 jaws and teeth by a horny beak, the tail shortened corre- 

 spondingly. The heaviest organs in the short trunk are lo- 

 cated as far as possible just beneath the line of support, join- 

 ing the sockets of the bones of the wings. The weight of 

 the body is lessened by reducing the least useful portions of 

 the vegetative organs. The respiratory organs are arranged 

 so as to supply oxygen to all parts of the body with the 

 smallest weight and least strain on the heart. 



The scales of reptiles have mostly become feathers, mar- 

 vellous structures, combining lightness, elasticity, durability, 

 and protection against cold and wet. The combination of 

 great energy and food-combustion with the warm covering 

 gives the body a constant temperature some ten degrees Fahr- 

 enheit above that of higher mammals. There are no sweat 

 glands to reduce it. We expect a very high development of 

 the brain. Here we are somewhat disappointed. In intel- 

 ligence, at least, the higher mammals seem decidedly superior. 



The eggs of birds relatively to the weight of the adult body 

 are much larger than those of reptiles. But the young are 

 usually born entirely incapable of caring for themselves and 

 are tended, fed and protected by the parents for several 

 weeks. They are few in number; in certain shore-birds only 



