12 HOMOIOTHERMISM 



frayed into feathers, which later spread over the whole 

 body. In early stages of this evolution, the climbing 

 fingers became elongated, and thus favored wing devel- 

 opment. With the growth of wings a keel for muscular 

 attachments projected more and more from the breast 

 bone. The hind toe of birds undoubtedly originated in 

 connection with adaptation to arboreal life. Heilman 

 concludes : 



The accelerated metabolic process, finally, produced an increased 

 caloricity, protected by feathering, until the warm-blooded state was 

 attained. The air-sacs of the lungs have expanded, spreading through 

 the whole body and filling the bones with air. The increase of all 

 these activities, moreover, has also resulted in a considerable enlarge- 

 ment and a somewhat refining evolution of the brain. 



In this way the reptile, through millions of years and innumerable 

 generations, has been changed into a bird. 



Modern birds generally maintain a constant body 

 temperature somewhat above that of mammals. This 

 varies very little at different seasons of the year (Simp- 

 son, 1911) and is quite similar in primitive and special- 

 ized birds (Simpson, 1912). However, Stoner (1926) 

 has recently reported that the temperature of the adult 

 bank swallow, Eiparia riparia (Linnaeus) may vary as 

 much as 7 deg. C. and that of the young, 10 deg. C. 

 Kendleigh and Baldwin (1928) made similar observa- 

 tions on house wrens. Atkins (1909a) observes that the 

 osmotic pressure of a developing hen's egg rises from 

 5.5 to 7.3 atmospheres. The latter figure is about the 

 same as that for bird's blood. **The view is put for- 

 ward that birds are descended from organisms with an 

 osmotic pressure of five atmospheres or less." The os- 

 motic pressure of the internal fluids in the bodies of 

 modern reptiles is about five atmospheres. 



