OKIGIN AND DISTRIBUTION OF MAMMALS 309 



Australian Region. 



We cannot close our account of the fossil mammal world of the southern 

 hemisphere without a glance also at Australia, of which the mammal fauna is 

 so strange compared with that of all the other continents. From the Tertiary 

 here we know only the single genus Wynyardia, of which the systematic 

 position cannot be satisfactorily determined on account of the lack of teeth. 

 In the Pleistocene, however, especially in caves, we find remains of mono- 

 tremes and a rather large number of marsupials, which in part attained 

 gigantic body-size. They belong exclusively to forms which are more or less 

 closely related to existing Australian genera, or arie even in part referable 

 to these genera. The Australian mammals are usually regarded as the 

 descendants of Mesozoic forms, which by isolation here were able to develop 

 undisturbed into great variety. Now, however, since the discovery of an 

 important series of fossil Polyprotoclontia and Diprotodontia in the Tertiary 

 of Patagonia, we must ask the questions, whether the Australian types may 

 not have been derived from these South American forms, and when they 

 arrived in Australia. These questions are certainly not easily answered. 

 The derivation of the Australian Dasyuridae from those of Patagonia presents 

 no difficulties, but it seems scarcely permissible to trace back the Hypsiprym- 

 nidae, Macropodidae and P halting eridae, as well as Phascolomys and Diprotodon, to 

 the Caenolestidae — only Thylacoleo may perhaps come from Ahderites ; for in the 

 Caenolestidae the molars are very unequal in size, while in the Australian 

 Diprotodontia they are equal, and, moreover, in the first the front molar is not 

 merely the largest, but also usually strongly differentiated. On the other hand, 

 it is indeed very unlikely that there should have been no genetic connection 

 between the Australian Diprotodontia and the South American Caenolestidae, while 

 this is obviously the case with the Australian and South American Dasyuridae. 

 The theory of a common origin has thus a certain justification. If the ancestral 

 forms arose in South America and thence spread to Australia, this emigration 

 can only have taken place across a very incomplete bridge of land, for otherwise 

 the Edentata, Litopterna and Notoungulata would also have reached Australia. 

 Nevertheless, it is not impossible that South America and Australia received 

 the marsupials from a region so far unknown to us. In any case it would be 

 a mistake to answer definitely at present the question as to the origin of the 

 Australian mammals. 



Conclusion. 



As we have seen, the northern hemisphere is of much greater importance 

 as a centre for the origin of the mammals than all other parts of the earth. 

 Even South America, which was so prolific during the Tertiary, is by no means 

 comparable, for it is not at all impossible that its mammal world originated 

 from forms of the northern hemisphere, although connections between North and 

 South America occurred only up to the middle of the Eocene and at the end of 

 the Pliocene. Africa was a real centre of evolution only during the Eocene, 

 and then merely for the Archaeoceti and the Suhungulaia, and even of these it 

 is by no means certain that they were not derived from the mammals of the 

 European Cetaceous. From the Oligocene onwards Africa was practically 



