CHANGES IN PHYSICAL GEOGRAPHY. 825 



in the latter part of the tertiary epoch is testified by the 

 Idaho fossils. And there is thus no difficulty in un(^er- 

 standing their presence in the rivers which have now cut 

 their way to the Pacific coast. 



The similarity of the crayfish of the Amurland and of 

 Japan is a fact of the same order as the identity of the 

 English crayfish with the Astacus torrentium of the Euro- 

 pean Continent, and is to be explained in an analogous 

 fashion. For there can be no doubt that the Asiatic 

 continent formerly extended much further to the east- 

 ward than it does at present, and included what are now 

 the islands of Japan. Even with this alteration of the 

 geographical conditions, however, it is not easy to see 

 how crayfishes can have got into the Amur-Japanese 

 fresh waters. For a north-eastern prolongation of the 

 Asiatic highlands, which ends to the north in the Sta- 

 novoi range, shuts in the Amur basin on the west ; while 

 the Amur debouches into the sea of Okhotsk, and the 

 Pacific ocean washes the shores of the Japanese islands. 



But there are many grounds for the conclusion that, in 

 the latter half of the tertiary epoch, eastern Asia and 

 North America were connected, and that the chain of the 

 Kurile and Aleutian islands may indicate the position of 

 a great extent of submerged land. In that case, the sea 

 of Okhotsk and Behring's sea may occupy the site of 

 inland waters which formerl}'^ placed the mouth of the 

 Amur in direct communication with the Northern Ocean, 

 just as the Black Sea, at present, brings the basin of the 



