1902.] AND ANCIENT GEOGRAPHY. 3l9 



geological evidence is very slender on account of our defective 

 knowledge of the Beringian countries. 



For our present purposes, this has the following meaning. The 

 Potamobiidcc of eastern Asia, the remnants of which are known as 

 the subgenus Ccmibaroides, had easy access to northwestern America 

 by way of the Beringian connection, from the beginning of the Upper 

 Cretaceous to the end of the Tertiary. Since Cavibaroides is to be 

 taken, as we have seen above (p. 288), for the more primitive group, 

 the migration must have been in an easterly direction. It cannot 

 have taken place in recent times, since this way is now rendered 

 impossible, and just this recent (or Pleistocene) interruption (prob- 

 ably connected with a change of climate) has separated the Asiatic 

 and American range of Potamobius. Before the middle of the 

 Cretaceous it was also impossible for the crayfishes to pass along 

 this line, since then this connection was not yet formed, and thus 

 we obtain a very important lower limit for this process in the dis- 

 persal of the PotainobiidcB about the middle of the Cretaceous period. 

 Consequently, the Potamobiidce may go back, in their geological 

 history, at least as far as this time. We shall see later that we are 

 able to define also an upper limit for the time of immigration into 

 North America. 



2. CONNECTION OF EASTERN ASIA AND AUSTRALIA. 



Another geographic postulate of the distribution of the Potamo- 

 biidce. and Parastacidce is the connection of east Asia, the region 

 of Cambaroides, with Australia, the main region of the family 

 Parastacidce. This same connection, from Farther India and south 

 China over the east Asiatic islands to north Australia, is suggested 

 by the distribution of the subfamily of the PotamonincE. 



Other zoogeographical facts point the same way. Pilsbry (1894, 

 p. xlv) says that eastern Asia and China, southward to Australia, 

 constitutes a great division in Helix distribution, and many other 

 writers have emphasized the close affinity of the fauna of Australia 

 to that of southeastern Asia, although this is true only for certain 

 groups of animals. The opposite opinion, which generally pre- 

 vails, that Australia is sharply isolated from the rest of the world 

 in its faunal relations, is founded chiefly on the highest forms of life, 

 the Mammals. Other groups of animals, which permit us to draw 

 conclusions in this respect, indicate clearly that a large part of the 



