55^ 



occupied by enemies. They spread until 

 they meet barriers, such as mountain 

 ranges, great bodies of water, or even 

 large deserts like the Sahara and the 

 Kalahari in Africa. If it were not for 

 such barriers animals and plants would 

 be more widely spread. 



Long ago some puzzling facts were 

 noted about the distribution of animals 

 and plants. Why should almost all native 

 mammals in Australia and nearby islands 

 be of a primitive type, simpler in struc- 

 ture than the mammals of other conti- 

 nents? There are many kinds but, like 

 the kangaroo, all have pouches in which 

 their young are kept for a time after 

 birth. Of all the very many kinds of 

 mammals found outside of Australia, 

 there is only one group that is pouched, 

 the opossums of North and South Amer- 

 ica. It puzzled biologists that the 

 pouched mammals should be limited al- 

 most entirely to Australia and its neigh- 

 boring islands. An explanation of this 

 puzzling fact can be found if we assume 

 that more complex animals come from 

 simpler forms. We can then explain this 

 unusual distribution somewhat as fol- 

 lows. In past ages there developed a 

 simple type of mammal with a pouch. 

 At this time, according to evidence of 

 the rocks, Australia was connected with 

 Asia and Asia with Europe. Since there 

 were no barriers to stop them, these 

 early pouched mammals could have 

 spread all through Australia, Asia, and 

 Europe. Then Australia was cut off 

 from the rest of the world by oceans. 

 Ages later, elsewhere in the world, a 

 type of mammal developed that did not 

 carry its young in a pouch. This form 

 gave rise to the many more advanced 



The Earth and Its Inhabitants Change unit x 



Fig. 505 An Australian koala carrying her off- 

 sprijjg. For a short while after it was born she 

 carried it in a pouch, (nature magazine) 



types of mammals we know. The same 

 change did not happen to occur in Aus- 

 traha. In Australia the pouched mammals 

 continued to live and to change in many 

 ways throughout the ages. But no mat- 

 ter how they changed in other respects, 

 they remained as pouched mammals of 

 one type or another. Many of the plants 

 of Australia are also different and more 

 primitive than those of the rest of the 

 world. 



On many of the remote islands of the 

 earth both the plants and animals are 

 different from the plants and animals in 

 other parts of the world. There are 

 many puzzling problems of distribu- 

 tion that still remain to be solved. 



Explaining puzzling biological facts. 

 The idea that living things are related 

 and descended one from the other has 

 developed from a study of fossils and 

 from l)reeding experiments. Fossils show 

 that animals and plants changed in past 



