37 



Mammals ana Mankind 



Characteristics and Reptilian Origin. Mammals are animals that have 

 hair. No others, except birds, are warm-blooded, and no others, except birds, 

 have coverings that so well conserve the heat of their bodies. Mammals have 

 lungs; their breathing is always aided by the diaphragm, a muscle that works 

 like a bellows. Their red blood cells, without nuclei when mature, are uniquely 

 efficient oxygen carriers. The brain is relatively large due to the great develop- 

 ment of the cerebral hemispheres. 



Except in the two egg-laying species the eggs are minute, are without shells, 

 and contain scarcely any yolk. Fertilization is always internal. The young de- 

 velop within the body of the mother, are born alive and are fed milk produced 

 by the mammary glands for which the class is named. While the embryo is 

 developing, it is surrounded by membranes formed on the basic patterns in- 

 herited from reptiles (Fig. 35.2). In the higher mammals, the placenta, a 

 modification of the chorion and allantois, is unique among all animals in its 

 provision for the developing young. 



Birds and mammals arose from different branches of reptiles early in the 

 Reptilian Age. Mammals increased in number slowly through that long period 

 of 70 million years or more. Towards its end, however, one of the most conse- 

 quential developments in the history of life was quietly appearing, the rise of 

 flowering plants. After that, there were flowering trees, with edible leaves, 

 seeds, nuts, and fruits. Times of good feeding had come. The flowering plants 

 spread, especiaUy the grasses, as lands were lifted, and through seasonal 

 changes many climates became more livable. Swamps dried and became 

 grazing lands. The Great Plains of North America were coming into existence, 

 and grass-eating hoofed animals spread over them. The evolution of mammals 

 quickened and broadened following that of the plants. The great Age of Mam- 

 mals had begun (Figs. 37.1, 37.2, 37.3). 



Mammalian Structures and Functions. For the structure and physiology of 



752 



