644 



ORGANIC EVOLUTION 



Fig. 25-18. The opossum is the oldest living mammal, 

 having originated in the Cretaceous and maintained 

 itself unchanged up to the present time. 



of the northern hemisphere. Austraha was 

 bridged through the Malay Archipelago 

 and the two Americas were connected 

 through Central America, just as they are 

 today, although during early Tertiary times 

 there was no land bridge at this point and 

 the two continents were separated. Evi- 

 dence for this is substantiated by the fact 

 that marine life in tlie Atlantic and Pacific 

 oceans in this region is very similar yet 

 sufficiently different to be accounted for 

 only by the formation of a relatively recent 

 land barrier which separated the two 

 oceans. 



The primitive mammals, which probably 

 resembled our present-day monotremes 

 and marsupials, spread over these land 

 bridges into Africa, South America, and 

 Australia. Later the placental mammals 

 evolved in the northern continents and, 

 because of their superior ability to cope 

 with the environment, were able to drive 

 out or destroy all of the primitive mammals 

 in these northern areas. However, before 



the placental mammals made their way too 

 far south, the sinking of the land connec- 

 tion between North and South America 

 below the surface of the oceans prevented 

 them from getting into the southern con- 

 tinent. Likewise, the land connections be- 

 tween Asia and Australia and to a lesser 

 extent between Europe and Africa gave 

 way, thus cutting off the possibilities of 

 further migrations southward. The primi- 

 tive mammals were thus isolated from their 

 more aggressive relatives and were able to 

 survive up to the present. For that reason 

 we see not only primitive mammals but 

 also other primitive groups of animals 

 ranging from insects to birds in the south- 

 ern continents. 



The fact that primitive mammals could 

 not withstand the onslaught of their more 

 recent placental relatives has been demon- 

 strated within historical times. Rabbits in- 

 troduced into Australia all but took over 

 the continent, outstripping the local mar- 

 supials at a tremendous pace. Only re- 

 cently it was estimated that they consumed 

 one-quarter of the country's pasturage in 

 spite of every effort of man to exterminate 

 them. This seems to be true of other north- 

 ern animals as well when introduced into 

 southern continents. One interesting excep- 

 tion to this apparent superiority of northern 

 fauna is the case of the opossum (Fig. 25- 

 18). This marsupial made its way back 

 north over the recent land bridge between 

 the two Americas and it has competed 

 rather successfully there with its aggressive 

 descendants. 



Another interesting case of animal dis- 

 tribution which can be explained in no 

 other logical way than by evolution is the 

 occurrence in widely separated parts of the 

 world of animals that are morphologically 

 similar yet geographically far apart. For 

 example, the camel family is represented 

 by the true camels in Asia and Africa and 

 by the llama and its relatives in South 

 America. How can one explain tlie exist- 

 ence of these closely related animals in 



