CHAPTER FORTY-NINE 



Characters 

 of Mam- 

 malia 



Egg-laying 

 mammals 



MAMMALS 



1. MAMMALS are warm-blooded animals, differing 

 from birds in lacking feathers and in having two con- 

 dyles, or articulations of the skull with the first vertebra. 

 The heart has four cavities, the right and left auricles 

 and ventricles, and the body is usually covered with 

 hair. The largest whales, fully 80 feet long, are the 

 bulkiest of all living animals ; but some mammals are 

 so small that they can climb a stem of wheat. The 

 group came into existence during the Mesozoic, and 

 persisted for ages without very much development. 

 With the dawn of the Tertiary era the development of 

 modern mammalian life began, to produce in the course 

 of three or four million years an enormous diversity of 

 types, many of them highly specialized and very re- 

 markable. Eventually man appeared, a mammal 

 capable of looking back on all this long history and in 

 some measure grasping its character and significance. 



2. The class Mammalia is divided into two subclasses, 

 the Prototheria or egg-laying mammals and the Euthe- 

 ria or viviparous mammals. To the former are referred 

 the fragmentary remains from the Triassic, which give 

 us the earliest indication of mammalian life. Their 

 egg-laying habits are of course only inferred, from their 

 general resemblance to reptilian types. Even so, we 

 should hardly have the courage to assume the former 

 existence of oviparous mammals, were it not for the fact 

 that such creatures still exist in the Australian region. 

 These living Prototheria constitute the order Mono- 

 tremata, and include the duckbill (Ornithorhynchus) 

 ("bird bill" in Greek) of Australia, and the so-called 

 spiny Anteaters (Echidna or Tachyglossus, and Zaglos- 



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