CHAPTER IX. 



THE REIGN OF MAMMALS. 



THE incoming of that highest order of animals in which 

 man himself, in so far as his physical nature is concerned, 

 takes his place, presents some features which, though not un- 

 paralleled in the history of other forms of life, are still very 

 striking. The modern Mammalia are somewhat sharply divided 

 into three very unequal groups. First, those which present in 

 their full perfection the property of producing fully developed 

 young, which is one of the distinctive characters of t ie class. 

 These are the Placental Mammals. Secondly, those in which 

 the young are produced in a very imperfect condition, apd are 

 usually nourished for a time in a marsupium or pouch. These 

 are hence called Marsupials. They are for the most part con- 

 fined to Australasia, though a few occur in America ; and are 

 decidedly inferior in rank to the ordinary mammals. Thirdly, 

 those in which there is a bird-like bill, and also certain bird- 

 like or reptilian peculiarities of skeleton and of the alimentary 

 canal. These are the Monotremes, represented by a very few 

 species in Australia and New Guinea. 



In geological history, so far as the facts are at present known, 

 the second group, that of the Marsupials, antedated the others 

 by a vast lapse of time. The Marsupials appear in the Trias, 

 near the beginning of the Mesozoic period. The Placentals 



