380 THE SONORAN REGION. [CHAP. 



In an earlier chapter 1 a list has been given of the leading 

 Distinctness mammalian families common to the two divisions 

 of the Region. o f Arctogaea, and since in the foregoing chapter it 

 has been shown that many of the peculiar American families 

 are more or less intimately related to some of those common to 

 the two areas, it is manifest that throughout the Tertiary period 

 eastern and western Arctogaea must have had a land-connection 

 towards the north, so that there was an interchange of the fauna 

 of the more northern districts. Those American types which 

 penetrated as far south as what is now the Sonoran area would, 

 however, naturally tend to become isolated, and thus develop 

 into the families which may be regarded as characteristic of that 

 region. So far, therefore, from this area being merely a part of a 

 so-called Nearctic region, there are indications that it was differ- 

 entiated from the Holarctic at a time when the existing zoological 

 regions of the eastern half of the Arctogaeic realm were still unde- 

 fined. Indeed from the community of the Pliocene fauna of 

 southern Europe, Asia Minor, Persia, northern India, and south 

 China, it seems probable that the only divisions of the Arctogaeic 

 realm that could have been attempted would have been into (i) a 

 Sonoran region, (2) a Holarctic region, comprising the northern 

 districts of America, Asia, and Europe, (3) what may be termed a 

 Mediterraneo-Oriental region, including southern Europe, north 

 Africa, and the whole of southern Asia ; and (4) a Malagasy 

 region, which would then, or perhaps somewhat earlier, have 

 included Ethiopian Africa. 



As to the amount of interchange which took place during 

 Dual Origin Tertiary times between the mammals of the eastern 

 of Groups. and western divisions of Arctogaea, and as to whether 



similar generic types may have been developed independently in 

 the two areas, it is almost impossible to arrive at any satisfactory 

 conclusion. The suggestion that Equus has thus been independ- 

 ently evolved in the two areas, has been already mentioned 2 , and 

 this idea receives support from some very remarkable observations 

 recently made on the invertebrates inhabiting certain European 

 and North American caves. 



1 Sttpra, p. 176. 2 Supra, p. 168. 



