674 - Heredity and Evolution 



Fig. 32-42. Recently delivered young of an opossum, gently removed from the marsupial pouch of the mother 

 without disturbing their attachment to the mammary nipples. Marsupials, lacking a placenta, deliver the young 

 in a very immature and helpless state. (Copyright, General Biological Supply House, Inc.) 



small, incompletely developed, helpless young 

 are placed in the marsupium, a pouchlike 

 infolding of skin on the mother's belly. Here 

 they establish an oral connection with the 

 nipples of the mammary glands (Fig. 32-42). 

 In the marsupium the young are carried, nur- 

 tured, and protected until they develop into 

 active animals almost ready to fend for them- 

 selves. 



Monotremes and all marsupials, except 

 the opossum, occur naturally only in the 

 Australian region. Here, however, the mar- 

 supials particularly have achieved consider- 

 able abundance and diversity. In Australia 

 one finds not only kangaroos, wombats, and 

 wallabies, but also marsupial "cats," "mice," 

 "shrews," "moles," "bears," and even a 

 pouched species resembling the wolf. Appar- 



ently these primitive mammals, which once 

 inhabited many regions of the ancient world, 

 escaped extinction in Australia because this 

 large island became isolated from the main- 

 land. The isolation occurred quite early, be- 

 fore the higher placentate types had reached 

 Australia. Consequently the evolving marsu- 

 pials of Australia, unlike those of other re- 

 gions, were not swamped out by competition 

 from increasing hordes of fitter (placentate) 

 mammals. 



The Eitheria, or Placental Mammals. 

 These higher forms, having a placenta, are 

 able to carry and nurture the embryos in 

 utero and to postpone delivery until the 

 young have developed to a relatively ad- 

 vanced state. Sometimes the offspring are 

 able almost immediately to run (horses), 



