PLANT AND ANIMAL PROGRESSION 53 



The bird is a very glorified reptile, still retaining the scales 

 and beak and claws, but adding enormously to the freedom 

 of the individual by the development of warm blood and 

 feathers and almost perfect adaptation to flight. 



Warm blood and constant body temperature maintained 

 by physiological activity of great complexity is, except for 

 the brain itself, the greatest of all nature's inventions. It 

 gives to the bird and mammal a considerable control over, 

 and independence of, the environment. Their movements 

 are much more free, and the potential for high-level intelli- 

 gence is enormously increased. In fact, taking the brain of 

 the apes and man as an example, one can readily see that 

 such a high-level organ could not function properly in a 

 body which was subject to the usual daily climatic fluctua- 

 tions of temperature which is the experience of all cold- 

 blooded organisms. Mammals evolved warm blood inde- 

 pendently of birds from a different reptilian ancestor. They 

 very definitely improved on all previous methods of em- 

 bryonic development by the invention of the placental 

 membrane, a foetal-maternal contact which very greatly 

 reduced the hazards of reproduction. 



A structural survey of the main kinds of plants and ani- 

 mals could be expanded in any degree of detail into many 

 volumes, since every body system shows a general progres- 

 sive evolution on the basis set up in this chapter. It is true 

 that one organism is not necessarily higher because it is said 

 to be more complex. On the basis of complexity, and in re- 

 gard only to the digestive system, cud-chewing cattle with 

 their four stomachs are higher forms than man. Various 

 criteria for comparison have been proposed by proponents 

 of progress in evolution: adaptation, dominance, specializa- 

 tion, power of expansion, approximation to man, and so 

 forth. However, the standards set above— especially, in- 

 crease in independence and sensitivity and the final appear- 

 ance of rising levels of understanding— are believable and 

 workable. 



