CHAPTER XXVI 



Class MAMMALIA 



I, Prototheria ; 2. Metatheria ; 3. Eutheria 



Birds and Mammals have evolved along very different 

 lines, Birds possessing the air and Mammals the earth, and 

 it is difficult to say that either class is the higher. But 

 apart from the fact, which prejudices us, that man himself 

 is zoologically included among Mammals, this class is 

 superior to Birds in two ways — in brain development, and 

 in the relation between mother and offspring. In most 

 Mammals there is a prolonged organic connection between 

 the mother and the unborn young, which may have been, 

 as Robert Chambers suggested, one of the conditions of 

 progress. It is also characteristic of Mammals that the 

 young are nourished after birth by their mother's milk, and 

 it has been suggested that the usually prolonged infancy 

 was one of the factors in the evolution of the humaner 

 feelings. It is certain at least that the carefulness and 

 sacrifice of the mothers has been one factor in the survival 

 and success of Mammals, and we may find in the term 

 Mammalia, which Linnaeus first applied to the class, a hint 

 of the idea that in the evolution of this class the mothers 

 led the way. 



General Survey of Mammals 



There are three grades of MammaUan evolution : — 

 A. The duckmole {Ornithorhynchus) and the spiny 

 ant-eaters {Echidna and Proechidna) differ very markedly 

 from all other Mammals. The young are hatched outside 



of the body ; in other words, the mothers are oviparous. 



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