CH. V] IMPORTED ANIMALS IX AUSTRALIA. 225 



alone they have persisted. In fact the Marsupials have 

 had the start in a country eminently suited to them and 

 have only been beaten in the struggle for existence in 

 regions subsequently settled in by them and therefore 

 perhaps less fit for their peculiar organisation. There 

 still remains the puzzle why some races of animals have 

 been able to establish themselves in Australia in spite of 

 the existing mammalian fauna, and why others have not. 

 The case of the rabbit shows that we cannot get out of 

 this difficulty by assuming that as much colonisation as 

 there was room for has as a matter of fact taken place. 

 On this view we should regard the presence of the 

 elephant as an unsuccessful attempt at colonisation. The 

 presence of comparatively speaking numerous placental 

 Mammals on the outskirts of the Australian region 

 renders it impossible to have resort to the hypothesis that 

 colonisation was at such rare intervals that only the 

 Dingo and the other few Australian placental Mammals 

 have had time to establish themselves. The only way 

 out of this difficulty which at present suggests itself is to 

 dwell upon the superior energy of the younger races of 

 the Palsearctic region. 



It is generally held that the desert of Sahara was an 

 open sea during the Miocene period ; and that this checked 

 the immigration or emigration of many purely terrestrial 

 forms, and that it therefore formed a boundary between 

 the Ethiopian and Palsearctic regions. This boundary is 

 not accepted by everyone certainly ; but it is by some. Mr 

 Blanford has pointed out 1 that the geological views which 

 1 Presidential address to Geological Society 1890. 



B. z. L5 



