The Mammals 



required In all land mammals. It Is the generally accepted 

 belief among scientists that the milk glands are an evolu- 

 tionary development of the sweat glands. 



The transition from reptiles to mammals has covered a 

 period of many millions of years. We learn much from the 

 fossil remains of many species of mammals that were unable 

 to continue In their struggle for existence and we also learn 

 much of the ancestors of many living species whose forms 

 have been changed by evolutionary processes to meet the con- 

 ditions of changing environments. Nature leaves her foot- 

 prints which connect the trail of the dim past to the present 

 so that we may know how many of these changes came to be. 



One of the most Instructive connecting links between 

 reptiles and mammals Is found In the living forms of the 

 duckbilled water mole heretofore mentioned and also of the 

 porcupine ant-eater, both of which are found only In Aus- 

 tralia and Its adjacent Islands. These animals are warm- 

 blooded, that Is they have developed four chambered hearts. 

 They have developed sweat glands to regulate the body tem- 

 perature. They have changed their scales Into hair and 

 quills, have only one excretory duct for the discharge of the 

 waste matter of the body, suckle their young like other mam- 

 mals, and yet they still lay eggs which are hatched somewhat 

 in reptile fashion. These are the first steps above the reptiles 

 yet they are the lowest order of the mammal group. Their 

 young are nourished from enlarged pores of the mother's 

 skin which secrete a milky substance. They furnish a living 

 example of Nature's first effort to nourish the young from 

 the mother's milk. 



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