4o6 GENERAL PRINCIPLES 



(by descent) they must also be related geographically; either 

 they still live in the land where they originated or else there 

 must have been a path along which they could migrate to the 

 place where they are now found. 



824. Hence, the present geographical distribution of animals 

 may throw light on the question of the origin of species. 



825. Australia is separated from Asia by a barrier which is 

 practically impassable for mammals, and geologists tell us that 

 this has been the case for a long time — since the Cretaceous 

 period. Now if species change in the course of time one would 

 expect to find the Australian mammals unlike those of Asia or 

 elsewhere. This expectation is fulfilled in a remarkable way. 

 All the mammals, except a few which we have reason to believe 

 were carried there, are Monotremes and Marsupials. These 

 are the most primitive mammals and no living forms are found 

 outside of the Australian region except the American opossum. 

 Fossil remains show that Marsupials were at one time wide- 

 spread, but evidently they were unable to contend with the 

 higher mammals and became extinct. In Australia no higher 

 mammals developed. On the other hand the Marsupials 

 developed in great variety; herbivores, carnivores, gnawers, 

 subterranean mole-like forms and tree-dwelling forms. 



826. Africa and South America are also somewhat isolated, 

 and here we also find peculiar faunas. It is not remarkable 

 that a peculiar species of any kind should be confined to a given 

 area, but where several similar species of a remarkable genus are 

 found in the same isolated region, and when, farther, fossils of 

 still other related forms are found in the same region, only the 

 hypothesis of a common origin offers a satisfactory explanation. 

 Numerous examples of this kind occur. Some examples are 

 the following: The kiwi-kiwis of New Zealand, the catarrhine 

 monkeys of the Old World and the platyrhine monkeys of the 

 New World, and the rheas of South America. The Edentates 

 of the Old and New Worlds are also of distinct orders. 



