128 



ANIMAL BIOLOGY 



Wallabies are the best known examples of the Australian Mar- 

 supials, and the Virginia Opossum is one of the few scattered 

 survivors in America of a group once widely distributed over the 

 Earth. (Fig. 88.) 



The method of reproduction of the Marsupials is unique. The 

 eggs hatch, as it were, within the mother's body where development 

 proceeds for a short time, nourishment being provided through 

 an atypical or very simple placenta. Then the young are born in an 

 exceedingly immature condition and make their way to a pouch on 

 the abdomen of the mother. Here they attach themselves to the 

 teats of the mammary glands and are nourished by milk. Even 

 after the young are well-developed the pouch serves as a refuge. 



3. Placentals 



The Placentalia, or Eutheria, comprises all the rest of the 

 Mammals from the lowly Insectivora, such as Gymnura and 

 Hedgehog, to the Primates, including Man. All nourish their 

 young before birth by means of a highly complex placenta that 

 makes possible the protracted development of the embryo under 



Fig. 89. — An Edentate. Texas Nine-banded Armadillo, Dasypus novem- 



cinctus texanus. (From Newman.) 



ideal conditions for nutrition and protection within the mother's 

 body: conditions necessary for the establishment of niceties of 

 structure and function as exemplified, for instance, by a larger and 

 better brain. Thus it is fair to say that the placenta and associated 

 embryonic membranes are in no small degree responsible for the 

 commanding position of the group in competition with other forms 

 of life. (Fig. 134.) 



The adaptive radiation of Placentals to nearly all types of 

 environments and modes of life — from Whales to Bats, and Moles 



