CHAPTER XXI. 



The Mammals. 



I HE mammals constitute the topmost branch In the tree 

 of animal life. They are all warm blooded, air breath- 

 ing, terrestrial animals except those that have forsaken the 

 land and returned again to their primordial home — the sea. 

 The whales and seals are representatives of this order and 

 these are still air-breathing, warm-blooded creatures. Their 

 early development takes place Inside the body of the mother 

 where most of the processes through which all former life 

 has passed are again repeated. After birth the young are 

 suckled or nourished from the fatty, or milky, secretions of 

 the mammary glands. These secretions have been distilled 

 from the food which the mother takes and from her body 

 tissues and developed Into a digestible form containing the 

 greatest amount of nourishment with the least digestive effort 

 for the young offspring. 



The overlapping scales of the fishes and reptiles have 

 given way and been changed into a coating of hair with which 

 the bodies of most mammals are generally covered. This 

 serves as a body protection and helps to keep the blood at an 

 optimum temperature. 



There^are three groups in the mammal class of animals, 

 namely : 



1 — Those whose body forms are completed within the body of the 

 mother. 



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