THE RISE OF THE MAMMALIA 121 



We see, therefore, that in the typical Mammal 

 three sets of organs are concerned in the peculiar 

 mode of reproduction: the placenta and its asso- 

 ciated structures, the mammary glands, and the 

 teeth. 



There exist, however, two groups of Mammals, 

 the Monotremata and the Marsupials, which possess 

 these characteristics either not at all or else in a 

 very much simplified form, and though it may be 

 true that this simplification is due to some extent, 

 especially in the Marsupials, to a secondary loss or 

 degeneration, yet there can be no doubt that these 

 animals have left the main Mammalian stem at a 

 period before the reproductive arrangements had 

 reached their present degree of perfection, and that 

 they have retained to some extent a primitive con- 

 dition when these arrangements were in course of 

 evolutionary development. 



The Monotremes are represented by two very 

 peculiar genera of animals, the Echidna and Platypus 

 (see Frontispiece), which exist to-day in Australia, 

 Tasmania and New Guinea, but nowhere else in the 

 world ; nor do they occur as fossils save in the recent 

 deposits of these countries, so that their geological 

 history is really unknown, though we must suppose 

 it to be a very ancient one. They possess the essen- 

 tial structural characters of Mammals ; thus they are 

 clothed with hair and they suckle their young with 



