CHANGES IN PHYSICAL GEOGRAPHY. 325 
in the latter part of the tertiary epoch is testified by the 
Idaho fossils. And there is thus no difficulty in under- 
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 formerly 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 
