756 EVOLUTION OF ANIMALS Part V 



Fig. 37.4. The joints of cats allow them great flexibility and grace of movement. 

 The turns of a cat's forefoot and leg during a face washing rivals those of a human 

 hand and arm in the same exercise. (Courtesy, Putnam: Animal X-Rays. New 

 York, G. P. Putnam's Sons, 1947.) 



New Guinea and Tasmania, but not in New Zealand as might be expected. 

 Marsupial moles (Notoryctes) and others inhabit South America; and the 

 opossum (Didelphis virginiana) is well known in our southern states (Fig. 

 37.8). The majority of marsupials are plant feeders; originally, they probably 

 all were; now there are carnivorous ones such as the Tasmanian wolf (Thyla- 

 cinus) which has been nearly exterminated because of sheep killing. 



Among the pouched mammals are mice, rats, squirrels, sloth-like "bears," 

 koalas, bandicoots that suggest rabbits with longer tails, and kangaroos. Brood 

 pouches are examples of convergence in evolution, the independent origin of 

 similar functions in genetically unrelated plants and animals. The male sea 

 horse, which is a fish with a broad pouch, and the female kangaroo, a mammal, 

 illustrate convergence. These animals are widely different and only distantly 

 related, yet both carry their young in pouches. 



Newborn marsupials are very small and immature. The great kangaroo, 

 Macropus major, is about 1 inch long when it is born and enters the pouch. 

 There it becomes attached to one of the nipples and milk is pumped into its 

 mouth by the contractions of muscles about the mammary gland. In this 

 kangaroo, the development before birth lasts for only 5 or 6 weeks. There is 

 little food in the egg and no adequate supply from the mother. After birth, the 

 young joey is carried in the pouch for about eight months. During the last part 

 of its stay, it leans out of the opening and sometimes crops grass as its mother 

 grazes, often jumping out and in again, reluctant to leave its carriage. 



Placental Mammals — Subclass 3, Placentalia. The members of this group 

 include all the other mammals, the cats, elephants, polar bears, and others 

 throughout the earth. There are about 3500 species of placental mammals in 



